This article appeared in Intelligence and National Security, 16 (Autumn 2001), 79-92, and may not be reproduced without its permission. This may be obtained from the administrative editor of the publishers, Frank Cass & Co., Ltd., Crown House, 47 Chase Side, Southgate, London N14 5BP, United Kingdom. Email to <editors @ frankcass.com>.
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AN HISTORICAL THEORY OF INTELLIGENCE
By David Kahn
Intelligence has been an academic discipline for half a century now. Almost from the start, scholars have called for a theory of intelligence. None has been advanced. Although some authors entitle sections of their work ※theory of intelligence,§ to my knowledge no one has proposed concepts that can be tested. I propose here some principles that I believe warrant being called a theory of intelligence because they offer explanations or predictions that can be seen to be true or untrue. I believe that the facts I offer
validate the theory; other scholars may adduce facts that disprove it.
I define intelligence in the broadest sense as information. None of the definitions that I＊ve seen work. It＊s like the term ※news.§ Though all but impossible to define, every journalist knows what it is: when something newsworthy is said in a court or a legislative hearing, all the reporters start taking notes.
My principles seek to deal with the past, the present, and the future of intelligence by accounting for the rise of intelligence to its current importance, explaining how it works, and specifying its main unsolved problems.
I The Past
The roots of intelligence are biological. Every animal, even a protozoan, must have a mechanism to perceive stimuli, such as noxious chemicals, and to judge whether they are good or bad for it. At that level intelligence is like breathing: essential to survival, but not to dominance. To this primitive capacity for getting information from physical objects, humans have joined the ability to get it from words. This verbal ability has led to a form of intelligence far more powerful than the kind used by animals or men to hunt prey or flee predators. It has driven the rise of intelligence to its present significance.
For intelligence has not always been as important or as ubiquitous as it is today. Of course, rulers in all times have used it, and have even paid tribute to it. Ramses II beat prisoners of war to make them reveal the location of their army. The Hebrews spied out the land of Canaan before entering it. Sun Tzu wrote, ※Now the reason the enlightened prince and the wise general conquer the enemy wherever they move and their achievements surpass those of ordinary men is foreknowledge.§ Ancient India＊s Machiavellian treatise on kingship, the Arthasastra, declares: ※My teacher says that between power (money and army) and skill in intrigue, power is better＃.No, says Kautilya, skill for intrigue is better.§ Caesar＊s legions scouted their barbarian foes. In the age of absolutism, ambassadors paid informants, while specialists in curtained, candle-lit black chambers slid hot wires under wax seals to open diplomats＊ missives 每 and then decoded them. Before the Battle of Prague, Frederick the Great observed his enemies＊ disposition from a steeple.
But these and similar episodes were sporadic. Most did not stem from an organized effort to gain intelligence. Usually, generals won battles without much more information about their foes than seeing where they were. Cannae, the classic victory of warfare, in which Hannibal encircled a larger Roman force and annihilated it, owed nothing to intelligence. Though rulers outlined campaigns, they did not detail mobilization and battle plans, giving intelligence little to discover. That is why, in fourteen out of Edward Creasy＊s Fifteen Decisive Battles of the World: from Marathon to Waterloo, victory was decided by strength, brains, and will 每 with knowledge of the enemy playing an insignificant role. The exception was the Battle of the Metaurus River in Italy in 207 B.C. The Romans, having intercepted a Cathaginian message, were able to concentrate their forces, defeat Hasdrubal before his brother Hannibal could reinforce him, and become the chief power of the Western worl.d.
The French and industrial revolutions begat new conditions. In creating the modern world, they created modern intelligence. The desire of Frenchmen to defend their new democratic nation against the invading armies of monarchist states, and the need to counter the professionalism of these states＊ forces with a superiority in numbers, led to armies far larger than those of the past. By 1794, France had a million men under arms. The lev谷e en masse called for a war economy to support it. Crops were requisitioned. Industrial output was nationalized. Suddenly, factors that had never counted in war became significant. It mattered little to a medieval king how much coal and iron his enemy could produce; such knowledge was vital to a modern head of state. Railroads made possible the rapid mobilization, concentration, and supply of large bodies of troops. These deployments called for war plans far more detailed than any ever envisioned by Caesar or Frederick. At last, intelligence had targets that gave it a chance to play a major role in war.
The industrial and political revolutions also expanded the sources that enabled intelligence to gain access to these new targets. I divide these sources into two kinds.
One consists of information drawn not from words but from things. It is seeing marching troops, fortifications, supply dumps, campfire smoke; hearing tank-motor noise, smelling cooking, feeling ground vibrations. I call it physical intelligence. For centuries it came only from the observations of the common soldier, patrols, the cavalry. But the balloon, the zeppelin, the airplane provided more physical intelligence more quickly than the deepest-driving horsemen. The camera saw more than the eye and reproduced its vision for others. Radar detected oncoming bombers long before the eye, even aided by searchlights, could spot them. In addition, larger armies meant more prisoners who might report on, say, the supply situation or artillery positions. All these sources provided more physical intelligence than armies had ever been able to get before.
But this increase was greatly outstripped by the growth of the second source, verbal intelligence. This acquires information from a written or oral source, such as a stolen plan, a report on troop morale, an overheard order, even a computerized strength report. Verbal intelligence made intelligence as important as it is today.
Verbal intelligence had long been relatively sparse. But the two revolutions engendered new sources. Larger armies yielded more documents for seizure. Parliamentary government, with its debates and public reports, exposed many specifics about a nation＊s military strength and programs. A daily press reported on these as well as on the economic situation. The tapping of telegraph wires and the interception of radio messages furnished far more verbal intelligence than the occasional waylaying of a courier ever did.
This growth is significant because verbal intelligence can furnish more valuable information than physical. Understanding this must begin with an acknowledgment that war has both a material and a psychological component. The material elements consist of such tangibles as troops, guns, and supplies. The psychological comprises such matters as a commander＊s will, his tactical ability, and the morale of his troops. The material factors dominate: the most brilliant, most determined commander of a regiment cannot withstand an army. And this factor is served by verbal intelligence, while the less important psychological component is served by physical intelligence. The reason is this: The men and weapons that are the sources of physical intelligence affirm the likelihood of an encounter with greater probability than a plan, for men cannot move guns or troops as easily as they can rewrite orders. Greater probability is another way of saying less anxiety, and anxiety is a psychological factor. Physical intelligence, by lessening anxiety, steadies command. On the other hand, verbal intelligence deals with intentions, and just as the enemy needs time to realize those plans, so a commander who knows about them gains time to prepare against them. He can shift his forces from an unthreatened flank to an endangered one, for example. In other words, verbal intelligence magnifies strength 每 or, in the current jargon, is a force multiplier. Thus it serves the material component of war, and because that component is the more decisive, verbal intelligence influences more outcomes than physical.
For the first 4,000 years of warfare, up to the start of World War I, nearly all information came from physical intelligence. That is why intelligence played a relatively minor role: physical intelligence does not often help commanders to win battles. Then, when the guns of August began firing, radio, which in effect turns over a copy of each of its messages to the foe, and the trench telephone, which lets indiscreet chatter be easily overheard, generated enormous quantities of verbal intelligence. These two new sources helped important commanders win important victories.
In August 1914, Germany＊s interception of a radioed plain-language Russian order told General Paul von Hindenburg and his deputy, General Erich Ludendorff, that they would have time to shift troops from a northern front in East Prussia, where the Russians were advancing slowly, to a southern one, where the Germans could outnumber them. The Germans made the move 每 and won the Battle of Tannenberg, starting Russia into ruin and revolution. In 1917, Britain＊s cryptanalysis and revelation of the Zimmermann telegram 每 in which Germany＊s foreign minister, Arthur Zimmermann, promised Mexico her ※lost territory§ in Texas, New Mexico, and Arizona if she would join Germany in a war against America -- helped bring the United States into the war, with all that that has entailed. It was the most important intelligence success in history. Britain＊s knowledge of Germany＊s naval codes enabled the Royal Navy to block every sortie of Germany＊s High Seas Fleet 每 and so keep it from winning the war in an afternoon.
Verbal intelligence served on the tactical level as well. It helped the Germans when, in 1916, the British fought to take the adjoining villages of Ovilliers and LaBoiselle on the Somme. The British suffered casualties in the thousands. In a captured enemy dugout, they found a complete transcript of one of their operations orders. A brigade major had read it in full over a field telephone despite the protest of his subordinate that the procedure was dangerous. ※Hundreds of brave men perished,§ the British signal historian related, ※hundreds more were maimed for life as the result of this one act of incredible foolishness.§
At last the admirals and generals understood. Intelligence had made its influence clear to them in the way they knew best. Despite their reluctance to share power and glory with intelligence officers, they realized that to spurn intelligence might cost them a battle or even a war 每 and their jobs. They and their governments drew the appropriate conclusions. Britain, Germany, Italy, and the United States, none of which had had codebreaking agencies before the war, established them after it. Germany, the most conservative state, whose General Staff had long subordinated intelligence to planning, created for the first time in its history a permanent peacetime military agency to evaluate all information. Intelligence had arrived as a significant instrument of war.
And in the next war, verbal sources made intelligence even more useful to commanders. It sped victory, saving treasure and lives. The reading of U-boat messages enciphered in the Enigma machine shortened the Battle of the Atlantic, the most fundamental struggle of the war, by months. Other Enigma solutions disclosed some of the Wehrmacht＊s tactical plans, particularly in France in 1944. Cracking the Japanese purple machine enabled the Allies to read, for example, the dispatches of the Japanese ambassador in Germany, giving them what U.S. Army Chief of Staff General George C. Marshall called ※our main basis of information regarding Hitler＊s intentions in Europe.§ The Battle of Midway, which turned the tide of the war in the Pacific, was made possible by intelligence from codebreaking. Marshall described its value:
※Operations in the Pacific are largely guided by the information we obtain of Japanese deployments. We know their strength in various garrisons, the rations and other stores continuing available to them, and what is of vast importance, we check their fleet movements and the movements of their convoys. The heavy losses reported from time to time which they sustain by reason of our submarine action largely result from the fact that we know the sailing dates and routes of their convoys and can notify our submarines to lie in wait at the proper points.§
Generals actually praised intelligence. Marshall said the solutions ※contribute greatly to the victory and tremendously to the saving in American lives.§ General Dwight Eisenhower wrote to the head of the British secret service, whose best information came from codebreaking, that ※the intelligence which has emanated from you ＃ has been of priceless value to me.§  Their tributes crowned the ascent of intelligence from its humble biological origins as a mere instrument of survival to its supreme capability: helping a nation win a war.
II The Present
The theory of verbal and physical intelligence explains, I believe, how intelligence grew 每 its past. But it also describes the present, by showing how physical intelligence steadies command and verbal intelligence magnifies strength. So I believe it can be incorporated into the principles that a theory of intelligence should offer. Indeed, it forms the first of three.
This first principle defines the function of intelligence. Magnifying strength and steadying command may be compressed into this: Intelligence optimizes one＊s resources. I call it O＊Brien＊s Principle, after Patrick O＊Brien, an economic historian, who casually remarked to me before lunch one day at St. Antony＊s College, Oxford, ※Well, David, isn＊t all intelligence just optimizing one＊s resources?§ This is the fundamental, the ultimate purpose of intelligence.
O＊Brien＊s Principle, like any logical proposition, may be obverted. A unit may not have intelligence and thus may not optimize its resources. It may be overwhelmed or, in intelligence terms, surprised. Surprise is the obverse of O＊Brien＊s Principle.
Another corollary of O＊Brien＊s Principle explains what a commander does when he has no intelligence, or faulty intelligence: I have dubbed this the null hypothesis. In the physical realm, he creates a reserve. The purpose of a reserve, Clausewitz said, is ※to counter unforeseen threats＃. Forces should be held in reserve according to the degree of strategic uncertainty.§ In the mental realm, the commander must remain firm in his decisions. He ※must trust his judgment and stand like a rock on which the waves break in vain＃. The role of determination is to limit the agonies of doubt.§ In other words, when a commander lacks the information that can optimize his resources, he must replace it with force and will. These are the counterparts of intelligence in the physical and psychological components of war.
The second permanent principle of intelligence holds that it is an auxiliary, not a primary, element in war. Some writers say loosely that intelligence has won this battle or that, but this is hyperbole. Battles and wars are won by men and guns, brains and will. Intelligence merely serves these. It is secondary to disposing one＊s forces, obtaining supplies, inspiring the troops. When I asked a general once whether he would rather have a good intelligence man on his staff or a good commander for one of his division＊s three regiments, he laughed, and his wife said that even she knew the answer to that one. The regimental commander, they said, was far more important. Colonel David Henderson, one of the first military men to study modern intelligence, declared in The Art of Reconnaissance that information cannot be classed with such matters as tactics, organization, discipline, numbers, or weapons because ※its influence is indirect, while theirs is direct.§ It is indeed a force multiplier and facilitator of command, but it cannot always make up for insufficient strength or inadequate leadership. It is a service, not an arm.
The third principle is perhaps the most interesting. It came to me when, while working on a book, I was looking for cases in which intelligence helped win battles. I noticed that I was finding many more defensive victories than offensive. These ranged from battles of worldwide importance, such as those of the Metaurus, Tannenberg, and Midway, to smaller operational clashes, such as the German Ninth Army＊s rebuff of a Soviet offensive south of Rzhev in November 1942 on the basis of all-source intelligence, down to tactical actions, such as the repulse of a Soviet counterattack out of Sevastopol on 21 January 1942, to which wiretaps had alerted the German 24th Infantry Division.  In all of these, intelligence helped award victory to the defenders. On the other hand, when intelligence helped win offensive victories, it rarely served directly, as by ascertaining enemy strength or intentions. Rather, as at D-Day, it aided deception 每 a doubly indirect service. Wondering why intelligence seemed to play so much more significant a role in the defense than in the offense, I looked up the definitions of these two modes to see if they offered a clue. Clausewitz＊s seemed to, and eventually I propounded a hypothesis. It maintained that intelligence is essential to the defense but not the offense. This theory seemed to explain several phenomena, suggesting that it might be valid.
Intelligence exists, of course, in both the offense and the defense, but in different ways. The difference is that between an accompanying and a defining characteristic. All elephants are gray, but grayness is not a defining characteristic of elephants, merely an accompanying one. Intelligence is a defining characteristic of the defensive; it is only an accompanying characteristic of the offense.
※What is the concept of defense?§ asked Clausewitz. ※the parrying of a blow. What is its characteristic feature? Awaiting the blow.§ Now, an army can await a blow only if it expects one, and it can expect one only on the basis of information or belief, right or wrong, about the enemy. There can be, in other words, no defense without intelligence. And Clausewitz says the same thing contrapositively when he asserts that surprise is needed for an offensive victory.
To defend is to acknowledge that the initiative comes from the enemy. And, indeed, the offense acts, the defense reacts. The offense prescribes to the enemy; it makes the basic decisions. It is ※complete in itself,§ said Clausewitz. Thus, information about enemy intentions, while helpful and to a certain degree always present (an army must see its enemy to fight it), is not essential to an offensive victory. An invading force can march about the countryside, imposing its will, without needing to know where the enemy is. (If it learns that the enemy plans to counterattack, it shifts to a defensive mode 每 and then it requires intelligence.) Military theorist Barry Posen has observed, in the terms of information theory, that the offensive, by seizing the initiative and thereby structuring the battle, reduces uncertainty (one reason commanders love it). And less uncertainty means less need for intelligence -- which, in one of its functions, steadies command.
What all of this says is this: While intelligence is necessary to the defense, it is only contingent to the offense. The validity of this principle is demonstrated, I believe, by two data. One is the relative frequency of defensive intelligence successes over offensive ones 每 the phenomenon that started me on the search. The second is that the nations that are aggressive tend to neglect intelligence, while nations in a defense posture emphasize and rely on it. A clear example is Poland between World Wars I and II. Her fear of being gobbled up by one or the other of her powerful neighbors motivated her 每 alone of all the powers 每 to crack the German Enigma cipher machine. Another case is Britain. She long based her foreign policy on the balance of power, which is a reactive technique; it needs intelligence to succeed 每 and Britain＊s secret services were legendary. An example that proves the irrelevance of intelligence to the offensive is Nazi Germany. Hitler expected to dictate (and for a while did dictate) to others as he began to conquer the world; for this he did not need intelligence, so he neglected his espionage and cryptanalytic organs to concentrate instead on Stukas, panzers, and elite divisions 每 and when the war came, his inadequate intelligence failed him. During the Cold War, the United States, worried about Soviet aggression, enormously extended its intelligence agencies. And the Soviet Union, almost paranoid about encirclement and subversion, developed the largest intelligence system on earth.
III The Future
These three principles of intelligence 每 it optimizes resources, it is an auxiliary function in war, and it is essential to the defense but not to the offense 每 seek to explain intelligence＊s operation and its place in the universe, just as the theory of the rise of verbal intelligence seeks to explain how intelligence became as important as it is. But what must intelligence do to improve? What problems must it resolve? What is its future?
I ask this in the largest sense. It is not a question of whether the end of the Cold War will decrease intelligence activity or whether the need to watch a multiplicity of nations, ethnic groups, economic institutions, and terrorists will increase it. Nor is it a question of techniques and their constant seesaw struggle with countermeasures. In response to the pervasiveness of this century＊s intelligence, a technology of stealth 每 the silent submarine, the bomber almost invisible to radar 每 has emerged. These, in turn, have given rise to ever more refined techniques of detection, such as instruments that spot the gravitational anomalies created by a mass of metal underwater. But none of these issues raise fundamental questions about the future of intelligence 〞 nor does the perennial difficulty that these sensors collect far more raw data than the agencies can evaluate in usable time.
Intelligence faces two all-encompassing, never-ending problems. Both are ultimately unsolvable. But intelligence must strive for a solution in the way that a graphed function reaches for 每 but never actually meets〞its asymptote.
The first problem is how to foretell what is going to happen. The goal, of course, is to predict everything. And certainly prediction is better in many cases that it ever was before. The new ability springs from the growth in intelligence tools. During World War II, Allied codebreaking revealed many more U-boat operations than it did in World War I. Wellington said, ※All the business of war ＃ is ＃ guessing what was at the other side of the hill.§  Today, the near blanketing of the theater of war with Buck Rogers collection devices 每 over-the-horizon radar, television cameras in the noses of drones and smart bombs, wide-ranging and detailed surveillance by satellite 每 renders the other side of the hill almost as visible as this side. It＊s hard to imagine an invasion like that of D-Day surprising any nation possessing today＊s observation tools. Still, not everything can be known in advance. Camouflage conceals men and weapons. Commanders change their plans. Accidents happen. These hindrances are multiplied a thousandfold in dealing not with a confined though complex activity like a single battle but with the major events of the post-World War II world, such as, for example, the fall of the shah of Iran. Many more factors, many more people come into play than in a limited action. Even without secrecy, the interaction of these elements is all but incalculable. As Clausewitz said of the difficulty of evaluating another state＊s capabilities and intentions, ※Bonaparte was quite right when he said that Newton himself would quail before the algebraic problems it would pose.§
This is why intelligence did not foretell North Korea＊s gamble in attacking South Korea, the Soviet emplacement of nuclear missiles in Cuba, the end of the Cold War itself. Prediction may be getting better, but it can never be perfect.
Even if it were, it would confront intelligence＊s other basic problem, which may be called the Cassandra complex: how to get statesmen and generals to accept information that they do not like. How can they be awakened to intelligence as the trumpet of a prophecy? This problem is as old as mankind. Pharaoh slew the bearers of ill tidings. Stalin ignored dozens of warnings that Germany was about to attack his country. Hitler swept aerial photographs from his desk when that indisputable evidence showed overwhelming enemy strength. The problem was clearly seen by Germany＊s pre-World War I chief of the General Staff, Count Alfred von Schlieffen: ※The higher commander generally makes himself a picture of friend and foe, in the painting of which personal wishes provide the main elements. If incoming reports appear to correspond with this picture, they are accepted with satisfaction. If they contradict it, they are discarded as entirely false.§ When Secretary of the Navy Knox was told that the Japanese had bombed Pearl Harbor, he said, ※My God, this can＊t be true. This must mean the Philippines.§
This condition, which psychologists call denial, is not limited to military or political affairs. People often reject reality. An investor does not want to hear all the reasons that a project may fail. A husband insists that his wife, coming home late, is faithful. As Rod Stewart sings (or croaks), ※Still I look to find a reason to believe.§ Shakespeare long ago set out the phenomenon in Troilus and Cressida: ※＃yet there is a credence in my heart, / An esperance so obstinately strong,/ That doth invert th＊ attest of eyes and ears.§ Edna St. Vincent Millay asked her readers to ※Pity me that the heart is slow to learn / What the swift mind beholds at every turn.§ And a little boy begged of Shoeless Joe Jackson, upon hearing that he had betrayed a World Series for money, ※Say it ain＊t so, Joe.§
Can this very human disposition be changed? Can the facts and logic of intelligence ever overcome wishful thinking? At present, they can only do so if the feelings are not deep-seated. If the consequence of facing the facts is too painful, the evidence will be ignored, suppressed, denied.
Where then is intelligence headed? A new factor darkens its future. Intelligence owes its success to the growth of verbal information. But as the cheap, miniaturized, unbreakable systems of cryptography proliferate, they will increasingly deprive cryptanalysts of the opportunities that data banks and the Internet and cellular telephones offer. As America＊s first modern cryptologist, Herbert O. Yardley, said in 1929 of AT&T＊s unbreakable one-time tape cipher machine, ※Sooner or later all governments, all wireless companies, will adopt some such system. And when they do, cryptography [codebreaking], as a profession, will die.§  The amputation of intelligence＊s right arm will cripple it. Just how serious this problem will be, however, no one yet knows.
But other factors counter this one and brighten the promise of intelligence. People see the advantage of permitting intelligence 每 in both its politico-military and its personal meanings 每 to rule emotion. They know that reason usually produces better solutions to problems than feelings do. This explains the growth of psychotherapy; this is part of what Paul meant in his profound statement to the Corinthians: ※For we cannot do anything against the truth, but only for the truth§;  this is why intelligence is so useful. As David Hume wrote in An Enquiry Concerning Human Understanding: ※We may observe, in every art or profession, even those which most concern life or action, that a spirit of accuracy, however acquired, carries all of them nearer to that perfection.§ He calls this ※the genius of philosophy,§ making intelligence a branch of that high domain, and says that from this accuracy, ※the politician will acquire greater foresight and subtlety,＃and the general more regularity in his discipline, and more caution in his plans and operations.§  Reason also produces technologies superior to those stemming from tradition or charisma, and these technologies allow their societies to dominate others. Witness the subjugation of China at the turn of the 20th century, the conquest of Native Americans, Europeans＊ grab of colonies in Africa, the rise of post-Perry Japan. The very establishment of intelligence agencies indicates a tendency toward greater reliance on facts and logic. The trend＊s success suggests that it will continue.
Accentuating this trend is an aspect of man＊s nature. Aristotle opened his Metaphysics by stating, ※All men by nature desire to know.§ The first man is the first example. Adam wanted to know what God told him he should not know, so he ate of the fruit of the tree of the knowledge of good and evil, and thus brought death, sin, and sorrow into the world. Like Adam, like Faust, every intelligence service strives to realize what the evangelist Luke put into words: ※For nothing is hid that shall not be made manifest, nor anything secret that shall not be known and come to light.§  None achieves it. ※It will always be a certain tragedy of every intelligence service,§ wrote the first head of Germany＊s post-World War I spy service, ※that even the best results will always lag behind the clients＊ desires.§ But the absence of perfection does not keep leaders, political and military, from letting intelligence serve them, any more than they let the absence of perfection keep then from using any other resource they have. Evidently they believe intelligence＊s results are worth its costs. Should they always follow its sometimes implied advice? No. It may be wrong. It is almost certainly incomplete. But they should at least take it into account.
The universal tendency toward least effort will further enlarge intelligence. As an optimizer of resources, intelligence saves money by reducing the need to buy military equipment 每 though, as merely an auxiliary element of war, it cannot reduce this need to zero. Since it is integral to the defense, intelligence will be increasingly seen as essential to nonaggressive nations. Yet it must improve its predictions and must convince leaders to accept them if it is to fully realize its potential.That potential spreads beyond the military. Like the benefactions of knowledge, of which it is a form, the benefactions of intelligence touch all humankind. In war, intelligence OPTIMIZES RESOURCES AND SO shortens the struggle, sparing gold and blood. In peace, it reduces uncertainty and so relaxes tensions among states, helping to stabilize the international system. These are the ultimate human goods of intelligence; these are the ways this servant of war brings peace to man.
 Sir Alan Gardiner, The Kadesh Inscriptions of Ramesses II (Oxford: University Press, 1960), 28-30.
 Numbers 13.
 Sun Tzu, The Art of War, trans. by Samuel Griffith (Oxford: Clarendon Press, 1963), 13:3.
 Kautilya＊s Arthasastra, trans. R. Shamasastry, 4th ed. (Mysore: Sri Raghuveer Printing Press, 1951), 367 (bk. IX, ch. i).
 The Gallic War, ii.17.
 The most detailed description of the workings of a black chamber, that of Austria, is by Harald Hubatschke in his 1973 dissertation for the University of Vienna, ※ Ferdinand Prantner (Pseudonym Leo Wolfram), 1817-1871: Die Anfänge des Politischen Romans sowie die Geschichte der Briefspionage und des Geheimen Chiffredienstes in Österreich,§ at 1269-1328, 1445-1460.
 Livy XXVII.xliii.1-8; Cambridge Ancient History, 8:91-96.
 It is important to understand that it is the source that matters, not the method of acquisition or the method of transmission. The presence of tanks can be ascertained by a spy and reported by telephone, but this information remains physical intelligence. Enemy plans can likewise be discovered by a spy and relayed by a photograph, but the information is verbal intelligence. The difference rests solely on the objects of intelligence themselves. Verbal objects mean verbal intelligence; nonverbal, physical intelligence.
 The Allies beat the Germans in World War II primarily because of their overwhelming material superiority. See John Ellis, Brute Force: Allied Tactics and Strategy in the Second World War (London: Andr谷 Deutsch, 1990).
 Of course physical intelligence can also reveal enemy capabilities, but doing so requires an inference 每 an extra step. Verbal intelligence reveals intentions without that mediation. To simplify, I have reduced this to the preceding formulation.
 Germany, Reichsarchiv, Der Weltkrieg: 1914 bis 1918 (Berlin: Mittler & Sohn), 2 (1925), 136-137, 351; Max Hoffman, War Diaries and Other Papers, trans.Eric Sutton (London: Martin Secker, 1929), 2:265-267, 332.
 David Kahn, ※Edward Bell and His Zimmermann Telegram Memoranda,§ Intelligence and National Security, 14 (Autumn 1999), 143-159, contains the latest scholarship.
 R. E. Priestly, The Signal Service in the European War of 1914 to 1918 (France) ([London?] 1921), 106.
 David Kahn, Hitler＊s Spies (Macmillan: New York, 1978), 418.
 United States, Congress, Joint Committee on the Investigation of the Pearl Harbor Attack, Pearl Harbor Attack, Hearings, 79th Congress (Washington: Government Printing Office, 1946), 3:1133.
 Eisenhower to Menzies, 12 July 1945, Eisenhower Library, Abilene, Kansas.
 This means successful intelligence, not perfect intelligence. Failed intelligence is not considered here as intelligence.
 Carl von Clausewitz, On War, trans.Michael Howard and Peter Paret (Princeton: Princeton University Press, 1976), 210.
 Ibid., 117, 102-103.
 I have discussed this more fully in ※Clausewitz and Intelligence,§ Journal of Strategic Studies, 9, nos. 2 and 3 (1985), 117-126, later republished as Clausewitz and Modern Strategy, ed. Michael I. Handel (London: Frank Cass, 1986).
 (New York: Dutton, 1907), 2.
 David Kahn, ※The Defense of Osuga, 1942,§ Aerospace Historian (Winter1981).
 Germany, Bundesarchiv-Militärarchiv, 24.Infanterie Divison, 22006/11, 19. Januar 1942, 22006/1, 21 January 1942; 50. Infanterie Division, 22985/4, 20 and 21 Januar 1942; Hans von Tettau und Kurt Versock, Geschichte der 24. Infanterie-Division 1933-1945 (Stolberg: Kameradschaftsring der ehemaligen 24. Infanterie-Divison, 1956), 24.
 Clausewitz, 357.
 Ibid., 198.
 Ibid., 524,
 Barry R. Posen, The Sources of Military Doctrine: France, Britain, and Germany Between the World Wars (Ithaca: Cornell University Press, 1984), 47-48.
 As Hamlet says, in a line generals would love, ※We defy augury§ (V.ii.23). For intelligence does not always resolve problems, does not always eliminate uncertainty. In a footnote above, I excluded failed intelligence from any definition of intelligence. But partial or erroneous information certainly exists in the world, and is sometimes included in the term ※intelligence.§ It is in this sense that one German officer explained that what went through generals＊ minds when the intelligence officer approached was ※Here comes the intelligence officer with his same old stuff. But I＊m going to do it like this anyway.§ (Kahn, Hitler＊s Spies, 415). Hamlet also maintained that uncertainty weakens determination. Referring to man＊s incomplete knowledge (in this case of death), he soliloquized, using ※conscience§ to mean thinking, ※Thus conscience does make cowards of us all, / And thus the native hue of resolution / Is sicklied o＊er with the pale cast of thought, / And enterprises of great pith and moment / With this regard their currents turn awry,/ And lose the name of action§ (III.i.83-88).
 George J.A. O＊Toole, ※Kahn＊s Law: A Universal Principle of Intelligence,§ International Journal of Intelligence and CounterIntelligence, 4 (Spring 1990), 39-46.
 This is developed at greater length in my Hitler＊s Spies: German Military Intelligence in World War II (New York: Macmillan, 1978), 528-531.
 John Wilson Croker, The Croker Papers, ed. Louis J. Jennings (London: John Murray, 1884), 3:275.
 Clausewitz, 586.
 Generalfeldmarschall Graf Alfred von Schlieffen, Gesammelte Schriften (Berlin: Mittler, 1913), 1:188.
 ※Pity Me Not,§ a sonnet.
 The American Black Chamber (Indianapolis: Bobbs-Merrill, 1931), 365.
 2 Corinthians 13:8.
 (1777), ∫1:4.
 Luke, 8:17.
 Fritz Gempp, ※Geheimer Nachrichtendienst und Spionageabwehr des Heeres.§ Im Auftrag der Abwehrabteilung der Reichswehrministeriums (U.S. National Archives microfilm T-77, Rolls 1438-1440, 1442, 1507-1509), II:7:162.
 George K. Zipf, Human Behavior and the Principle of Lease Effort (Cambridge, Mass.: Addison-Wesley, 1949).
Copyright © 2008 David Kahn. All rights reserved.