The intuition and deduction thesis as a rationalist account of a priori knowledge, the meaning of and distinction between intuition and deduction, Descartes' clear and distinct ideas, the cogito as an a priori intuition, the trademark argument and the proof of the external world as a priori deductions, and the empiricist objections that such reasoning yields either certainty without substance or circularity.

What this dot point is asking

AQA wants you to explain the intuition and deduction thesis: the rationalist claim that some substantive knowledge of reality can be gained a priori, by intuiting self-evident truths and deducing further truths from them. Explain how intuition and deduction differ, set out Descartes' clear and distinct ideas, the cogito as an a priori intuition, and the trademark argument and proof of the external world as a priori deductions, then evaluate the thesis against the main empiricist objections.

Intuition and deduction

The thesis is a form of rationalism: reason alone can deliver substantive truths about reality, against the empiricist claim that all substantive knowledge of the world is a posteriori. The foundations of knowledge are reached by reason, not observation. Mathematics is the model: we do not measure triangles to learn their angles sum to two right angles, we prove it.

Descartes' clear and distinct ideas

For Descartes, the engine of a priori knowledge is the clear and distinct idea. An idea is clear when present to an attentive mind, and distinct when sharply separated from everything else. His rule is that whatever I perceive clearly and distinctly is true: clarity and distinctness are the mark of truth. The programme depends on this criterion, where empiricists press hardest.

The cogito as an a priori intuition

The cogito is the fixed point from which knowledge is rebuilt, and it yields the criterion: it is certain because perceived clearly and distinctly, so Descartes generalises that clarity and distinctness is the mark of truth.

The trademark argument and the external world as a priori deductions

From the cogito and the criterion, Descartes proceeds by deduction:

• The trademark argument (the existence of God). Descartes finds in himself the idea of a supremely perfect being. By the causal principle that a cause must have at least as much reality as its effect, this idea cannot have come from a finite, imperfect mind; it must have been placed in him by such a being, like a craftsman's trademark. So God exists, an a priori deduction from the content of an idea.
• The proof of the external world. Because God is perfect, God is no deceiver. We have a strong, involuntary inclination to believe our perceptions are caused by external objects. If that belief were systematically false, God would be a deceiver, which is incompatible with perfection. So the external world exists, and our clear and distinct ideas of its primary qualities are reliable.

AQA tests the structure: an intuited foundation (the cogito), a criterion of truth (clarity and distinctness), then a chain of a priori deductions (God, then the external world).

Evaluating the thesis

The thesis promises certainty and a secure foundation, and mathematics seems to show reason can deliver necessary truths a priori. But three objections press hard.

• Hume's fork. Every truth is either a relation of ideas (a priori but analytic, empty of information about the world) or a matter of fact (synthetic but a posteriori). If so, the thesis's a priori claims are either trivially analytic or really empirical in disguise. There is then no synthetic a priori knowledge of reality, and the programme collapses.
• The Cartesian circle. Descartes uses clear and distinct perception to prove God, then uses God to guarantee clear and distinct perceptions are true, which is circular.
• The criterion is unreliable. "Clear and distinct" is too subjective: false beliefs have seemed clear and distinct to many, so clarity is no guarantee of truth.