Commentary on the Posterior Analytics

Overview

About four hundred pages in most modern editions, which print Aristotle’s text interleaved with Aquinas’s commentary lecture by lecture. Aquinas composed it near the end of his working life, probably around 1271 to 1272, part of a series of commentaries he wrote on Aristotle’s major treatises during the final, most productive years before his death in 1274.

The work being commented on, Aristotle’s Posterior Analytics, is the second half of what became known as the Prior and Posterior Analytics, together forming the core of the logical works later compiled as the Organon. Where the Prior Analytics sets out the formal theory of the syllogism, the mechanics of valid inference, the Posterior Analytics asks a further and harder question: what makes a piece of reasoning not merely valid but genuinely scientific, productive of certain knowledge rather than merely correct deduction from whatever premises happen to be given? It is widely regarded as one of the most difficult texts in the entire Aristotelian corpus, compressed to the point of obscurity in places, and Aquinas’s commentary functions as an indispensable guide through it, though it is also, in its own right, a significant statement of Aquinas’s own theory of knowledge and science.

Most programs that assign this text assign only the Proemium, Aquinas’s introduction, which runs a few pages and stands reasonably well on its own as a compact statement of how medieval thought classified the different modes of reasoning and located demonstrative science among them.

The argument

What wisdom orders, and why logic comes first. The Proemium opens with a striking claim, borrowed from Aristotle’s Metaphysics: the office of the wise person is to order things, because wisdom is reason operating at its most powerful, and ordering is precisely what a powerful reason characteristically does, whether ordering the parts of a single science, one science in relation to another, or human acts and things in relation to their ends.

From this Aquinas derives a justification for beginning philosophical education with logic. Because human beings acquire knowledge of the truth through the activity of reason, and because logic is the science that studies the very act of reasoning itself, considering how reason proceeds from what is already known to what was not yet known, logic functions as an art directing the other sciences, teaching the method by which all of them must proceed if they are to yield genuine knowledge rather than mere opinion or error.

The four operations of reason. The heart of the Proemium is a fourfold classification that orients everything the rest of the commentary will discuss. Reason, applied to the same subject matter, can proceed in four distinct ways, differing in the certainty of the conclusions they yield.

Demonstration produces conclusions that are wholly certain, because it proceeds from premises that are themselves necessarily true and known to be the actual causes of the conclusion. This is the domain of the Posterior Analytics specifically, and Aquinas treats it as the highest and most reliable mode of reasoning available to the human intellect.

Dialectic produces conclusions that are only probable, because it proceeds from premises that are merely generally accepted or endorsed by the wise, rather than premises whose truth is grasped with certainty. Dialectical reasoning, covered in Aristotle’s Topics, is genuinely valuable, particularly for examining and testing opinions, but it cannot deliver the certainty demonstration delivers.

Rhetoric produces conclusions that are merely persuasive, aiming to move an audience toward belief or action through means, including the character and credibility of the speaker and appeals to emotion, that fall short of demonstrating anything is actually true.

Sophistical reasoning has only the outward appearance of valid reasoning without the reality, deliberately or through error mimicking the form of a good argument while actually committing a fallacy, covered in Aristotle’s Sophistical Refutations.

Locating the Posterior Analytics within this fourfold scheme tells the reader exactly what standard to expect from it: not persuasion, not mere probability, but the strictest and most demanding standard of certain, causally grounded knowledge that human reasoning can achieve.

What counts as scientific knowledge. Following Aristotle, Aquinas defines scientific knowledge, scientia, precisely: we are said to know something scientifically when we know it through its cause, know that this cause could not be otherwise, meaning the connection is necessary rather than accidental, and know that the conclusion follows necessarily from that cause. All three conditions together are required; missing any one of them means what you have is not scientific knowledge in this strict sense, however confident or even correct the belief might happen to be.

This sets up the commentary’s extended treatment, following Aristotle through the body of the Posterior Analytics, of what the premises of a genuine demonstration must look like: they must be true, primary, immediate, better known than and prior to the conclusion, and, crucially, they must state the actual cause of the conclusion rather than some merely coincidental fact that happens to be true alongside it.

Definition and demonstration. A substantial portion of the commentary works through Aristotle’s difficult treatment of how definitions relate to demonstrations. Aquinas explains Aristotle’s view that a proper scientific definition of a thing states its cause, so that defining a thing correctly and demonstrating a property of that thing correctly are two expressions of essentially the same underlying grasp of why the thing is what it is. A definition that merely lists observable features without capturing causal structure, on this account, falls short of what a genuine scientific definition requires.

How first principles are known. The commentary’s final movement addresses a problem Aristotle raises and that generated enormous later discussion: if demonstration always proceeds from prior premises, and those premises would themselves need to be demonstrated from still prior premises, either the process regresses infinitely, which would mean nothing is ever actually demonstrated, or it must terminate somewhere in premises that are not themselves demonstrated but are known some other way.

Aristotle’s answer, which Aquinas explains carefully, is that the most basic first principles of a science are grasped not through further syllogistic reasoning but through a distinct cognitive process involving repeated sense experience, from which the intellect, through a power Aristotle calls nous and Aquinas discusses at length, comes to grasp universal principles directly and non-inferentially. This account of how the human mind moves from particular sensory encounters to grasp of universal, necessary truths became a central reference point for later medieval and scholastic epistemology.

How to read it

Read the Proemium on its own first, ideally more than once. It is short, it stands independently of the rest of the commentary, and it gives you the entire conceptual map, the four operations of reason, the definition of scientific knowledge, the location of demonstration among its neighbours, that everything else in the commentary presupposes.

Read Aristotle’s own text of the Posterior Analytics alongside Aquinas’s commentary rather than the commentary alone. Aquinas is explaining a text, clause by clause, and the commentary’s value depends on seeing exactly what difficulty in Aristotle’s compressed Greek each passage of Aquinas is working to resolve.

Do not expect quick progress. This is dense, technical philosophy of science and logic, closer in spirit and difficulty to a modern work in epistemology or philosophy of science than to Aquinas’s more narrative or theological writing, and it rewards slow, careful reading with a pencil rather than continuous reading for pleasure.

Pay special attention to the account of how first principles are grasped through experience rather than through further demonstration. This solves, or attempts to solve, one of the oldest problems in the theory of knowledge, the regress problem, and it is worth working out in your own words exactly how Aquinas thinks repeated sensory experience can yield grasp of a genuinely universal and necessary truth.

Read the Division and Methods of the Sciences alongside this commentary if you want the fuller picture of how Aquinas organizes knowledge. That shorter text addresses how the different theoretical sciences are distinguished from one another; this commentary addresses what makes reasoning within any one of them genuinely scientific in the first place, and the two texts complement each other closely.

Questions it raises

• Is Aquinas’s fourfold division of demonstration, dialectic, rhetoric and sophistry exhaustive, or are there modes of reasoning it leaves out?
• Does the strict definition of scientific knowledge, certain, causal, necessary, describe any actual modern science, or does it set a standard too demanding for empirical inquiry as it is actually practiced?
• If first principles are grasped through repeated experience rather than demonstrated, how confident can we be that what experience yields is genuinely necessary and universal rather than merely a strong habit of expectation?
• What is gained by insisting that a proper definition must state a thing’s cause, rather than simply describing its observable features accurately?
• Does treating rhetoric and dialectic as inherently less reliable than demonstration undervalue the genuine cognitive work persuasive and probable reasoning do in situations where certainty is unavailable?
• How much of Aquinas’s confidence in the possibility of certain, demonstrative knowledge of the natural world would survive contact with later scientific method, which typically treats even its best-established theories as revisable?