# What is the role of deductive reasoning in Euclid’s method of proceeding? Give an example.

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### 1 Answer

Euclid used deductive reasoning to prove his *Optics* theorems. Deductive reasoning is basically using many statements is a sort of 'top-down' logic to prove theorems.

Now what Euclid did in Opticswas that he used many postulates and definitions to prove the theorems about vision.

Some of the postulates in *Optics* are:

Let it be assumed :

1. That rectilinear rays proceeding from the eye diverge indefinitely;

2. That the figure contained by a set of visual rays is a cone of which the vertex is at the eye and the base at the surface of the objects seen;

3. That those things are seen upon which visuals rays fall and those things are not seen upon which visual rays do not fall;

4. That things seen under a larger angle appear larger, those under a smaller angle appear smaller, and those under equal angles appear equal;

5. That things seen by higher visual rays appear higher, and things seen by lower visual rays appear lower;

6. That, similarly, things seen by rays further to the right appear further to the right, and things seen by rays further to the left appear further to the left;

7. That thing seen under more angles are seen more clearly.

Using these postulates, Euclid proved his theorems about optics. This methodology is called **Deductive Reasoning**.