  Despite the necessary connection the mind has with the body, argued Spinoza, sensation and imagination are inherently limited. The idea of substance qua substance must be a perfect unity. However, the idea which constitutes the human mind is complex—not a unity but a plurality of ideas. That idea is therefore confused, rather than clear and distinct. Clear and distinct understanding, on Spinoza’s account, must partake of the unity of the idea of substance, and not of the fragmentary nature of the idea of the human body and its affects. This cognitive unity is achieved in two ways—through reason (which Spinoza termed “knowledge of the second kind”) and through intuition (“knowledge of the third kind”). When we cognize through sensation and imagination (“knowledge of the first kind”), we try to grasp many ideas at once, and thereby produce confusion. Reason and intuition, by contrast, provide us with access to just one idea—the substantial unity underlying our body and our mind. Reason does this from “the fact that we have common notions and adequate ideas of the properties of things”, while intuition proceeds “from an adequate idea of the formal essence of certain attributes of God”. To understand the substantial unity that is the necessary cause of our body and our mind is to grasp them sub specie aeternitatis.