CHAP. IV.: The Essence of a thing cannot be attained by Syllogism - Aristotle, Posterior Analytics 
Aristotle’s Posterior Analytics, trans. E.S. Bouchier, B.A. (Oxford: Blackwell, 1901).
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- Introductory Note.
- Book I.
- Chap. I.: Whether a Demonstrative Science Exists
- Chap. II.: What Knowing Is, What Demonstration Is, and of What It Consists
- Chap. III.: A Refutation of the Error Into Which Some Have Fallen Concerning Science and Demonstration
- Chap. IV.: The Meaning of ‘distributive,’ ‘essential,’ ‘universal’
- Chap. V.: From What Causes Mistakes Arise With Regard to the Discovery of the Universal. How They May Be Avoided
- Chap. VI.: Demonstration Is Founded On Necessary and Essential Principles
- Chap. VII.: The Premises and the Conclusion of a Demonstration Must Belong to the Same Genus
- Chap. VIII.: Demonstration Is Concerned Only With What Is Eternal
- Chap. IX.: Demonstration Is Founded Not On General, But On Special and Indemonstrable Principles; Nor Is It Easy to Know Whether One Really Possesses Knowledge Drawn From These Principles
- Chap. X.: The Definition and Division of Principles
- Chap. XI.: On Certain Principles Which Are Common to All Sciences
- Chap. XII.: On Questions, And, In Passing, On the Way In Which Sciences Are Extended
- Chap. XIII.: The Difference Between the Demonstration and Science of a Thing’s Nature and Those of Its Cause
- Chap. XIV.: The Figure Proper to Demonstrate Syllogism
- Chap. XV.: On Immediate Negative Propositions
- Chap. XVI.: On Ignorance Resulting From a Defective Arrangement of Terms In Mediate Propositions
- Chap. XVII.: On Ignorance Resulting From a Defective Arrangement of Terms In Immediate Propositions
- Chap. XVIII.: On Ignorance As Resulting From Defective Sense Perception
- Chap. XIX.: Whether the Principles of Demonstration Are Finite Or Infinite
- Chap. XX.: Middle Terms Are Not Infinite
- Chap. XXI.: In Negations Some Final and Ultimate Point Is Reached Where the Series Must Cease
- Chap. XXII.: In Affirmations Some Final and Ultimate Point Is Reached Where the Series Must Cease
- Chap. XXIII.: Certain Corollaries
- Chap. XXIV.: Whether Universal Or Particular Demonstration Is Superior
- Chap. XXV.: That Affirmative Is Superior to Negative Demonstration
- Chap. XXVI.: Direct Demonstration Is Superior to Reduction Per Impossible
- Chap. XXVII.: What Science Is More Certain and Prior, and What Less Certain and Inferior
- Chap. XXVIII.: What Constitutes One Or Many Sciences
- Chap. XXIX.: Concerning Many Demonstrations of the Same Thing
- Chap. XXX.: On Fortuitous Occurrences
- Chap. XXXI.: Sense Perception Cannot Give Demonstrative Science
- Chap. XXXII.: On the Difference of Principles Corresponding to the Difference of Syllogisms
- Chap. XXXIII.: The Distinction Between Science and Opinion
- Chap. XXXIV.: On Sagacity
- Book II.
- Chap. I.: On the Number and Arrangements of Questions
- Chap. II.: Every Question Is Concerned With the Discovery of a Middle Term
- Chap. III.: The Distinction Between Definition and Demonstration
- Chap. IV.: The Essence of a Thing Cannot Be Attained By Syllogism
- Chap. V.: Knowledge of the Essence Cannot Be Attained By Division
- Chap. VI.: The Essence Cannot Be Proved By the Definition of the Thing Itself Or By That of Its Opposite
- Chap. VII.: Whether the Essence Can In Any Way Be Proved
- Chap. VIII.: How the Essence Can Be Proved
- Chap. IX.: What Essences Can and What Cannot Be Proved
- Chap. X.: The Nature and Forms of Definition
- Chap. XI.: The Kinds of Causes Used In Demonstration
- Chap. XII.: On the Causes of Events Which Exist, Are In Process, Have Happened, Or Will Happen
- Chap. XIII.: On the Search For a Definition
- Chap. XIV.: On the Discovery of Questions For Demonstration
- Chap. XV.: How Far the Same Middle Term Is Employed For Demonstrating Different Questions
- Chap. XVI.: On Inferring the Cause From the Effect
- Chap. XVII.: Whether There Can Be Several Causes of the Same Thing
- Chap. XVIII.: Which Is the Prior Cause, That Which Is Nearer the Particular, Or the More Universal?
- Chap. XIX.: On the Attainment of Primary Principles
- Appendix. Prior Analytics. Book II.
- Chap. XXIII.: On Induction
- XXIV.: On Example
The Essence of a thing cannot be attained by Syllogism
A syllogism could only be expressed as a definition if a middle term convertible with the two other terms were employed. This, however, would involve a Petitio Principii.
Can a syllogism or a demonstration treating of a thing’s Nature be arrived at, or, as our recent argument assumed, is that impossible? Syllogism proves one thing of another by means of a middle term, but a thing’s Nature is a property and is predicated as part of its essence. Further, definition must be convertible with the thing defined; for if A be a property of B, and that again of C, each term is a property of the other. Further, if A be an essential attribute of B, and B be essentially universally and distributively predicable of C, A must be essentially predicable of C.
If, however, one do not thus make use of A twice over, it will not of necessity be predicable of C; that is to say when A is an essential attribute of B, but not of everything of which B is predicable. Both A and B then form part of the essence of C. Hence too B is essentially predicable of C; but if both A and B be essential attributes of the subject C, and also of the formal cause of C, the formal cause will be present in the middle term before the syllogism is formed.
In general then if one have to prove, e.g. What man’s essence is; let C be ‘man,’ and A the essence, whether this be ‘biped animal’ or something else. If then a syllogism is to be formed, A must be predicated distributively of B, and further a middle term is required, and this will be predicable of the essence A. Thus one will be assuming what one ought to prove, seeing that B will also denote man’s essence.
One ought to consider this phenomenon both in the two premises and in the primary and ultimate proposition, as it will appear most plainly in them. In fact, those who prove by means of a convertible proposition what is the essence of Soul, of Man, or of any other existing thing, are guilty of begging the question.
Thus, suppose it were asserted that the soul is a thing which is the cause of its own life, and that this cause is a number which moves itself. Here one would have to assume that the soul is like a number which moves itself, and is actually identical with it. A will not in fact be essentially predicable of C because A is a consequence of B, and B of C, though it may be conventionally so denoted; nor yet, if A exists, is it thereby made essentially and distributively predicable of B.
For instance, the essence ‘animal’ is predicated of the essence ‘man,’ and it is true to say that every essential attribute of man is an essential attribute of animal, (just as it is true to say that every man is an animal) but not in the sense that man and animal are identical.
If the terms are not so stated one cannot infer that A constitutes the essential nature and substance of C. If they are so stated there will be a preliminary assumption that B, the thing which ought to be proved, constitutes the essential nature of C. Thus no demonstration of this latter fact will have been given, but we shall have begged the question.