Mid-September 2026, school restarted a couple of weeks ago: in Year 4 philosophy picks up where the syllabus takes it every year, from Aquinas to Descartes, a September-October block and — in almost all classes — the first serious oral test of the year. It is not a chapter like the others: it is the gateway to modern philosophy, and if you do not fix methodical doubt and the cogito well now, you arrive in February struggling with Kant, who on his own takes up 4-6 weeks and is built precisely on what Descartes leaves open.
For many people “Descartes” boils down to one phrase — I think, therefore I am — repeated without knowing what it proves and what it leaves out: it is a chain of arguments that holds together step by step, and in the oral test it is immediately clear who has reconstructed it and who has only read it once.
What the October oral test really asks
The module on Descartes, in Year 4, usually includes the method, the three steps of doubt, the cogito, the classification of ideas, the proof of the existence of God, the Cartesian circle and the res cogitans/res extensa dualism. A serious question touches at least four of them, and often starts from a trap question like “Is Descartes a sceptic?” — it seems a detail, and instead it checks whether you have understood the structure, not just the vocabulary.
If you stopped at the bare fact (“Descartes doubts everything and then finds the cogito”), you answer the first question and get stuck on the second: you have the conclusion in mind, not the sequence that leads to it.
The mistakes that cost the most marks
Treating “cogito ergo sum” as a slogan. The cogito does not prove “that I exist”: it proves that I exist as a thinking thing. Body, outside world, other people all remain in doubt at the very moment of the cogito.
Confusing methodical doubt and sceptical doubt. It is the most frequent mistake of all — there is a dedicated section below.
Treating the three steps of doubt as interchangeable. Error of the senses, dream and evil genius are three levels of increasing radicalism, each leaving less standing than the one before: listing them without this order loses the point of the argument.
Not placing the idea of God in the classification of ideas. Many know that there are innate, adventitious and factitious ideas, but cannot say which category the idea of God belongs to, nor why it is decisive for the proof of his existence.
Not knowing the Cartesian circle. It is exactly the question with which a teacher separates those who have repeated the sequence from those who have understood its logical structure.
Why you need a method, even before doubting
Descartes is a mathematician before being a philosopher — analytic geometry, the one with Cartesian axes, is his — and his project is born from envy of the certainty of mathematics. The philosophy of his time, he says, is made of endless disputes where every thesis finds an equally defensible opposite; a geometric proof, once understood, is no longer disputed. To bring the same certainty into philosophy you need a method, not just good will.
In the Discourse on Method it is reduced to four rules:
- Evidence — never accept anything as true unless it is known clearly and distinctly, avoiding haste and prejudice.
- Analysis — divide every complex problem into as many parts as are needed to solve it better.
- Synthesis — conduct thoughts in order, from the simplest to the most complex, one step at a time.
- Enumeration — final reviews so complete as to be certain of having left nothing out.
Doubt is not a whim: it is the application of the first rule. If an idea does not pass the test of absolute clarity and distinctness, it has to be suspended — even if it seems obvious.
Methodical doubt, step by step
Doubt proceeds through three levels, each more radical than the last. It is worth reconstructing it in a table, because that is where the logic of the argument shows.
| What is put in doubt | With what argument | What remains standing |
|---|---|---|
| The most uncertain sense perceptions (a stick that looks broken in water, a tower that looks round from a distance) | Error of the senses: whoever has deceived me even once does not deserve blind trust | The very close and evident perceptions (I am sitting here, I have this sheet in my hand) |
| Even those very close perceptions, and my own body | Dream argument: there is no certain sign to tell waking from sleep | The simplest and most general truths — number, extension, figure — seem true both awake and in a dream |
| Even mathematics and the most elementary logic | Evil genius: an omnipotent, deceiving being who employs all his cunning against me | Apparently nothing — except one point, the subject of the next section |
The evil genius is not narrative colour: Descartes needs it to be sure of having doubted everything that can be doubted, without laziness. Only once he has hit bottom can he check whether anything remains that not even this extreme hypothesis manages to dent.
Methodical doubt or scepticism? The difference the oral test checks
This is where the point that almost everyone gets wrong is decided. Sceptical doubt — that of the ancient philosophers who suspend judgement on everything — doubts in order to remain in doubt: suspension is the goal. Descartes’ doubt doubts in order to get out of it: it is a tool, used only once within the meditation, to find a fixed point on which to rebuild knowledge — Descartes himself compares it to Archimedes’ point of support.
That is why we speak of methodical (or hyperbolic, pushed beyond the reasonable on purpose) doubt: it is a means, not an end. Answering “Descartes is a sceptic because he doubts everything” means having seen the surface and not the function of the argument — and that is exactly what this question checks.
The cogito: the only certainty that holds even under the evil genius
Even granting that an evil genius deceives me about everything — world, body, arithmetic — there is a fact that this deception makes necessary: for me to be deceived, I must exist. You cannot deceive someone who is not there. And doubting, being deceived, believing falsely are ways of thinking (Descartes uses “thought” in a very broad sense: understanding, willing, imagining, doubting, feeling are forms of cogitatio). Reconstructed as an argument:
- Premise 1: the evil genius can make me believe false any content of my thought — world, body, mathematics.
- Premise 2: but to be deceived, or even just to doubt, I must exist at that instant.
- Premise 3: doubting, being deceived, believing are acts of thought.
- Conclusion: therefore, every time I think — even just because I doubt or am deceived — my existence as a thing that thinks is necessarily true.
This is the cogito: not “I think, therefore I am” as an isolated sentence, but the end point of the chain. A clarification worth an extra mark: Descartes himself made clear that it is not a syllogism with a hidden major premise (“everything that thinks exists”) — it is grasped in a single act, an immediate intuition of the mind about itself. If the teacher asks “is it a syllogism?”, the answer is no, even though reconstructing it as an argument helps in understanding why it holds.
The point that almost everyone skips: the cogito guarantees only the existence of the self as res cogitans, a thinking thing — not that the body exists, nor that there is a world outside the mind. That part stays suspended, and it will take the rest of the Meditations, going through God, to get it back.
Innate, adventitious, factitious ideas: the road to God
To get out of the solipsism of the cogito, Descartes starts again from an observation: in the mind I find ideas of different kinds.
- Adventitious ideas: they seem to come from outside — the idea of the sun as I perceive it by looking at it.
- Factitious ideas: I construct them myself by combining pieces of other ideas — a hippogriff, a mermaid.
- Innate ideas: they come from my own nature as a thinking thing — the idea of a perfect triangle, of truth, and above all of God.
The idea of God does the decisive work: it has a content — an infinite, perfect, eternal being — out of proportion to me, finite and imperfect (I doubt, so something is missing in me). An effect cannot contain more reality than its cause: it is the principle that supports the proof. I, finite, cannot be the cause of an idea with infinite content: it must have been put in me by a being that is really, not just in idea, infinite. So God exists, and created me with this idea impressed on me — like a craftsman’s mark left on his work.
Once it is proved that God exists and is by definition perfect, the conclusion follows by itself: a perfect being does not deceive, because deceiving is a defect. This eliminates the evil genius hypothesis and guarantees that everything I perceive clearly and distinctly is true. God becomes the guarantor not of the single idea, but of the very criterion of truth.
The Cartesian circle: the objection that separates those who have understood
Here is the question that, more than any other, separates those who have reconstructed the argument from those who have only studied it. The objection — raised already by a contemporary of Descartes, Antoine Arnauld — is this: to prove that God exists, Descartes uses the certainty of clear and distinct ideas; but to guarantee that clear and distinct ideas are always true, he uses the existence of God. It is a circle: the criterion of truth proves God, and God guarantees the criterion of truth.
If the circle were perfect, the whole construction would collapse: you cannot use a conclusion to justify one of its own premises. The most quoted defence distinguishes two moments: perceiving a clear and distinct idea in the very act of thinking it (the cogito, every step of the proof of God) needs no external guarantee, because the certainty is immediate; the problem arises only for the memory of having perceived it in the past — and that is where God is needed. It is a plausible reply, but how far it really solves the problem remains an open debate among historians of philosophy — which is why being able to explain it well shows the level at which the game is played.
Res cogitans and res extensa: the problem Descartes leaves open
From the cogito and the rehabilitation of the external world, Descartes derives two radically distinct substances: the res cogitans, thinking thing, whose only attribute is thought; and the res extensa, extended thing, whose only attribute is occupying space. I have a clear and distinct idea of myself as a thinking, non-extended thing, and of the body as an extended, non-thinking thing: since I can conceive them separately, and what I conceive clearly and distinctly God can bring about, mind and body are truly two distinct substances — and the mind can, in principle, exist without the body.
The price of this distinction is a problem never fully resolved: if thought and extension have nothing in common, how does a bodily sensation (a thorn pricking) produce a mental state (pain), and how does a mental decision move an extended body? Descartes proposes the pineal gland as the point of contact, but already Princess Elisabeth of Bohemia, in a letter to Descartes, points out that indicating a physical place of contact does not explain how two such different natures can influence each other: it moves the problem, it does not close it. From then on the mind-body relationship remains one of the permanent knots of modern philosophy.
How to answer in the oral test in three minutes: an example
Here is how a solid answer, of about three minutes, to the question “why is Descartes’ doubt not scepticism?” might sound:
“Descartes’ doubt and scepticism are alike only on the surface, because both call our certainties into question. The difference is in the aim. The sceptic doubts in order to stay in doubt: the suspension of judgement is the end point, meant to achieve peace of mind. Descartes, by contrast, doubts in order to find something that doubt itself cannot dent, and to use it as a foundation to rebuild knowledge: that is why we speak of methodical doubt, a tool, not a conclusion.
The doubt proceeds through three increasing steps: first the senses, which sometimes deceive; then the dream, because there is no way of being certain that I am not dreaming; finally the extreme hypothesis of the evil genius, who could deceive me even about mathematics. It is a hyperbolic doubt, pushed deliberately beyond all reasonableness, because Descartes wants to be sure of having doubted everything that can be doubted.
Right at this extreme point, though, Descartes finds something that resists: if I am doubting, or am deceived, I must exist at that instant, because doubting and being deceived are forms of thought. That is the cogito. From here Descartes starts again to rebuild, step by step, everything he had put in doubt. The sceptic never starts again: he stays fixed at the suspension of judgement. Descartes uses doubt like scaffolding, which is taken down once the building is finished.”
This is the length and level of detail that holds up for three minutes of oral test without slips: definition, the steps in brief, the point that resists, and the difference from scepticism stated plainly.
How we work on this, in lessons
With students tackling Descartes in Year 4, the work with the tutors in the humanities area always follows the same order: first the reconstruction of the argument as a chain of premises and conclusion — as in the table and list above — and only afterwards the fluent formulation for the oral. Whoever learns the logical sequence does not need to memorise the textbook paragraph: he reconstructs it even if the question arrives in a different order from the manual’s.
The second thing we train is the trap questions — “is he a sceptic?”, “does the cogito prove that the body exists?”, “do you know the Cartesian circle?” — because they are the ones on which the difference between a 6 and an 8 is decided, and whoever has heard them once before does not freeze when they are asked for real.
Lessons are online, one-to-one, one hour at a time: for the current rates (single lesson, 10-hour package) see the pricing page, and before that there is a free 30-minute introductory call. Payment is offline — bank transfer, Satispay, PayPal, card, Apple Pay or Revolut. The full picture of how we set up the course in philosophy, history and Italian is on the page about the humanities subjects.
If in Year 4 besides philosophy you also have maths or physics to keep an eye on, you will find a mock test for each topic of the syllabus in the maths and physics tests for high school — for now they cover only these two subjects, not philosophy, but if in October you also have a calculus or mechanics test it is worth trying them: the correct answer is free, the guided solution unlocks with the first lesson.
To understand where the problem of the foundation of knowledge comes from, look at the year before: the Presocratics and the arche. To see where modern philosophy leads, in Year 5 there is Hegel’s dialectic.
If methodical doubt and the cogito are still a question mark two or three weeks from the test, write to us: we look together at your class’s syllabus and tell you honestly where it is best to restart.
