Spinoza's proof for Proposition 2 of part I of the Ethics, "Two substances whose attributes are different have nothing in common", is confusing. This is the proof:
1, prop 2, demo - Also evident from Def. iii. For each must exist in itself, and be conceived through itself; in other words, the conception of one does not imply the conception of the other.
The proof looks like Spinoza simply proves the following proposition 2*: "Two substances have nothing in common", as the proof only refers to the definition of substance and does not refer to Def. iv, which is the definition of attribute. So it must be evident for Spinoza, that proposition 2 and proposition 2* are equivalent. This equivalence seems to involve that it goes without saying that two substances have different attributes. Nonetheless, Spinoza delivers a proof for this conclusion in proposition 5: "There cannot exist in the universe two or more substances having the same nature or attribute." I conclude that either proposition 2 and 2* are not equivalent or the proof for proposition 5 should better be located before proposition 2, because 2 seems to effectively make use of 5. Furthermore, it seems strange to put forth some proposition as evident in one place (i.e. that substances can only have different attributes in 2) and delivering a proof for the same proposition in another place (i.e. in 5), indicating by this that the proposition is not that evident at all. How can one explain these difficulties of proposition 2?
