Spinoza never gives a single, stand-alone definition of “reason” as a dictionary entry. Instead, its meaning emerges from how it functions within his epistemology, psychology, and metaphysics—especially in the Ethics. A reasonably full account has to situate reason within that system.

1. Reason as a kind of knowledge

Spinoza distinguishes three kinds of knowledge (Ethics II, prop. 40, schol. 2):

1. Imagination (or opinion)
   Knowledge from sense perception, memory, testimony, and random experience. It is fragmentary, confused, and often false.

2. Reason (ratio)
   Knowledge from adequate ideas, especially common notions—ideas of what is common to all bodies or to the human body and other bodies. This knowledge is necessarily true.

3. Intuitive knowledge (scientia intuitiva)
   Immediate grasp of the essence of a thing as it follows from the essence of God or Nature.

Reason, then, is not just “thinking carefully.” It is a mode of knowing defined by the adequacy and necessity of its ideas.

2. Adequate ideas

Spinoza defines an adequate idea as one which, considered in itself, has all the properties of a true idea (Ethics II, def. 4). An idea is adequate when it explains its object through its causes, not merely through its effects on us.

Reason operates only with adequate ideas. This is why reason cannot err: falsity belongs to imagination, not to reason.

3. Common notions

The core material of reason is common notions (Ethics II, props. 37–39). These are ideas of features that are:

• Common to all bodies (e.g., extension, motion, rest), or

• Common to the human body and other bodies.

Because these features are always present and understood through their causes, ideas of them are necessarily adequate. Reason proceeds by forming and combining such notions.

4. Reason and necessity

Reason apprehends things sub specie aeternitatis—under the aspect of eternity. That is, it understands things not as contingent or accidental, but as necessary consequences of Nature’s laws.

To understand something by reason is to see:

• why it must be so,

• how it follows from the nature of things,

• and that it could not have been otherwise.

This is why Spinoza repeatedly contrasts reason with imagination’s tendency to see things as contingent, good or bad, purposive, or divinely intended.

5. Reason and the affects

Reason is not opposed to the emotions in the way later Enlightenment thinkers sometimes imagined. For Spinoza:

• An affect ceases to be a passion and becomes an action when we form an adequate idea of it (Ethics III, prop. 3).

• Reason does not suppress affects; it reorders them by understanding their causes.

Through reason, we form active affects such as rational desire, generosity, and self-contentment. Freedom, for Spinoza, consists in being determined by reason rather than by confused passions.

6. Reason and ethics

Spinoza’s ethical claims are grounded in reason:

• Reason teaches what is truly useful to us (Ethics IV, prop. 26).

• It shows that cooperation, justice, and benevolence follow necessarily from human nature properly understood.

• Moral notions are not commands but conclusions drawn from rational understanding of our essence as finite modes striving to persist (conatus).

In short

For Spinoza, reason is the faculty by which the mind forms adequate ideas—especially common notions—and understands things as necessary consequences of Nature’s laws. It is objective, causal, and non-teleological. It does not speculate about purposes or values imposed from outside the world; it explains how things must be, and how we, as part of Nature, can live more actively and freely within that necessity.