Spinoza's Ethics: II.D3: Idea

Spinoza moves first to a definition of idea to focus on what a mind does - it is a thinking thing, a mode, and its modal expression is the formation of ideas. The idea is formed as a concept of the mind as by the mind's action or power, not as a passive impression by an object. If the mind were passive to objects, the attribute of thinking would simply be a subset of the attribute of extending. To be separate but equal within substance, both attributes need to independently express the essence of substance.

This separation between object and idea seems foreign. Spinoza is not completely separating idea from object, because both idea and object are deeply connected through the mode itself that gives rise to them. The mode itself which expresses itself through either its extending aspect or its thinking aspect.

An example from childhood might make this "separate but equal but still the same in reality" more clear. Occasionally children discuss the relative merits of seeing versus hearing. Each aspect of gathering sensory data has its own characteristics. Neither informs the other in a dominant way. They are separate but equal in gathering information differently about the same underlying reality. 

The emphasis on action or power of the mind not only maintaining the unitary operation of God's expressive force within both attributes of thinking (IID3) and extending (IID1) but also holds significant implications for blessedness as the Ethics unfolds.

Given this unitary structure of Spinoza (in contrast to Descartes' dualism), consider the contrast to II.D1. There, the body is a mode under the aspect of extending. Here, the mind is similarly a mode under the aspect of thinking. So, if the mind forms ideas under the fact that it is a thinking thing, what does the body do under the fact that it is an extending thing? It attempts to persevere and extend its existence.

Thinking is different than Knowing in the same way as extending is different than Being or Existing. Thinking and extending are simply perspectival attributes and mode-based. Knowing and Being are conceptual and substance-based. This distinction of conceiving versus perceiving has held since ID4. The mind referred to here is not the operation of individual human minds, but mind as modal expression of the attribute of thinking.

Per ideam intelligo mentis conceptum quem mens format propterea quod res est cogitans.

Explicatio: Dico potius conceptum quam perceptionem quia perceptionis nomen indicare videtur mentem ab objecto pati. At conceptus actionem mentis exprimere videtur.

Translated as,

By idea, I understand a concept of the mind which the mind forms on account of the fact that the thing [mind] is [a] thinking [thing].

Explanation: I say concept rather than a perception because the name perception seems to indicate that the mind is acted on by an object. But concept seems to express an action of the mind.