The Search After Truth has many virtues—including scathing, robust critiques of Aristotelian/scholastic/medieval approaches to physics and natural phenomena—but it really can’t be said that brevity and concision are among them. The following is a paragraph-by-paragraph summary of Book Six, Part Two, Chapter Three.
The general rule concerning the subject of our studies. That the school philosophers do not observe it, which is the cause of several errors in physics.
 ‘We must reason only on the basis of clear ideas.’ Hence start with and dwell on what is maximally simple before proceeding to anything complex, and do so only on the basis of the former.
 Yeah, sure, you say—this may even strike you as obvious. But maybe you’ll be surprised to hear that almost nobody (read: the scholastics) actually follows this rule.
 Aristotle is the absolute master in this regard. (Malebranche says, words dripping with scorn, that he ‘justly deserves the status of Prince.’) Most of his ‘concepts’ are actually just vague terms that, because they are equivocal, don’t clearly and distinctly represent anything. The Physics is the worst: it says nothing, means nothing. There will turn out to be two kinds of equivocal terms to be distinguished.
 The first are equivocal is because the terms involved ‘arouse only sensible ideas’—i.e., they refer to sensation. Doing any work on their basis leads invariably to error.
 NB some equivocal terms aren’t really a problem: nobody mistakes the constellation for the animal when diagnosing an allergic reaction to fish. But ‘terms of sensible ideas’ are equivocal in much sneakier ways.
 So: ‘fire is hot’ in the scholastic mouth means ‘fire contains what I feel when I am warm’; ‘grass is green’ means ‘grass has on it the colors I believe I see there’. Thus they confuse modes of bodies with those of minds.
 Descartes teaches us to make the proper distinction here. If ‘heat’ is ‘the movement of insensible parts of bodies’, then the fire is hot. But if ‘heat’ is ‘what I feel near fire’, then the fire is not hot—rather, I am. Of course some people think that what they sense is the same thing that is in the object. But then everything falls apart, and you’ll never make your way back to clarity. Here’s why.
 First. Things feel differently to different people, and even to the same person at different times. You think this is bitter, I think it’s sweet—how can bitterness and sweetness both be in the same thing (at the same time and in the same way)? (My new favorite example of this, because maximally tragic, is the taste of cilantro.) This room seems hot to me because I’ve just come in out of the cold, but also cold when I’ve just gotten out of a hot shower—how can it itself be both hot and cold? Classic paradoxes of sensate becoming.
 Second. We abstract the sensation confusedly. Paper, sugar, and salt are all ‘white’. Is it the same ‘whiteness’ in each thing? Then its universality suggests that it is ‘in us’, and yet it must be ‘in the thing’ if our determination of that thing is to be valid.
 Third. Some nearly identical bodily qualities cause nearly opposite sensations; some radically different bodily qualities cause nearly identical sensations. The difference between tickling and torture has got to be one only of degree. Directly identifying the quality in itself and for us leaves us without a way to resolve this sort of problem.
 So people judge sensible qualities on the basis of sensations; then they determine objects based on these judgments of sensible qualities based on sensations. How could this not be a mess? Wheat and flour are determined to be essentially different because the sensations we have of them are quite different. But flour is just ground wheat. And so on.
 Upshot is that ‘terms for sensible ideas are completely useless for the distinct proposal and clear resolution of questions, i.e., for the discovery of truth.’ But Aristotle and the Aristotelians propose their questions and try to resolve them precisely in and through using just these kinds of equivocal terms.
 This approach has (by the 1600s) congealed into a kind of philosophical common sense. Ask the Aristotelian whether water is moist or fire is hot; they’ll answer immediately, having consulted nothing but ‘the impressions these objects have made on their senses, or those which their reading has left on their memory. They will not see that these terms are equivocal.’ M says: this common sense leads them to multiply distinctions that confuse things, and leave admixed the things that ought to be distinguished.
 Now if we subtract everything based on such equivocations, most of Aristotle and the scholastics falls away. But the same isn’t true of Descartes, who ‘explains the principle effects of nature in a clear, obvious, and often demonstrative manner using the only ideas that are distinct, i.e., those of extension, figure, and motion, the principle effects of nature.’
 So much for terms that are equivocal because they have to do with sensation. There is another category of equivocal term, which have to do with logic. Malebranche’s enumeration shows these to be transcendentals: ‘genus, species, act, potency, nature, form, faculties, qualities, cause in itself, and accidental cause.’ The Aristotelians don’t even realize that these words refer to nothing real. Worse, and this gets to the real heart of the problem: they have no real explanatory power. Hence ask the scholastic how and why we digest food, and they’ll invoke our ‘digestive faculty’.
 Those who go down this route are typically not wrong—because they’re taking no chances, since there’s nothing at stake in building propositions out of nothing-terms.
 ‘Fire heats, dries, hardens, and softens because it has the faculty to produce these effects.’ Bread nourishes because it has the nutritive quality. You can’t go wrong! But also, you’re not saying anything; you’re turning the question directly into the answer. Why/how does x do y? Because x has the y quality. ‘These or similar ways of speaking are not false: it is just that in effect they mean nothing. These vague and indeterminate ideas do not involve one in error, but they are completely useless for discovering truth.’
 We can see how useless this sort of claim is when we try to obtain new knowledge with one. Say we grant a substantial form of fire that involves a host of faculties like heating. Will exposure to fire lead to hardening or softening a given object? There’s no necessary connection if the question is posed in this way: fire hardens mud, but softens wax. Experience gives the answer, but that experience also has nothing to do with any such substantial form or causal faculties.
 Contrast this with the Cartesian claim that extended bodies are modified by the rates of movement and the organization of the particles that constitute them. For there is a connection between the hardness/softness of mud or wax, and the heat of fire—hardness, softness, and heat are all understood in terms of particulate velocity, and thus can be meaningfully related. This means that the knowledge involved in understanding fire in this way has predictive value (e.g., I can predict that exposure to fire will soften wax and harden mud), whereas there is nothing of the sort in Aristotelian-scholastic positing a ‘dilating faculty’ or ‘hardening faculty’.
 This is what happens when you posit substantial forms in your reckoning with natural phenomena. And there’s no stopping point: you’ll posit a substantial form or causative faculty for every sensation you receive. This is like throwing Ockham off a thousand cliffs (just one would have been sufficient).
 This gets us back to the rule: reason only on the basis of simple, clear ideas. ‘If we ask the philosophers what sort of entity is fire’s illuminating faculty, they will answer only that it is the being that causes fire to be capable of producing light. So their idea of this faculty is no different from the general idea of cause and the confused idea of the effect they see.’ There just are no clear ideas to be found here. Physics demands better.