In this article I will consider Spinoza's argument proposed in the Second Scholium of Proposition 8, which argues for the impossibility of two substances sharing the same nature. I will begin by explaining in detail the two-step structure of the argument and proceed accordingly by offering a structured account of its relationship to the main claim. Therefore, I will point out what I reasonably consider to be an error in Spinoza's reasoning; that is, the definition of a thing does not express a fixed number of individuals according to that definition. In contrast, I hope to motivate the claim that a true definition of a thing actually expresses a fixed number of individuals who fall within that definition. I will then present a difficulty against my view and concede its insufficiency to block Spinoza's conclusion. Finally, I will resort to a second objection in an attempt to prove an example by which two substances contain a similar attribute, but differ in their nature. In view of these considerations, I conclude that Spinoza's thesis is erroneous. In the argument of the Second Scholium on proposition 8, Spinoza makes two assertions which have the following form: First, there is necessarily for each individual existing thing a definite cause which explains its existence. existence. Second, the definite cause of the existence of a thing must either be contained in the very nature and definition of the thing existing - in which case the existence belongs to the nature of the thing defined - or be postulated externally to its definition and its nature. . If, for example, there were to exist five women in the universe, or several individuals of the same species, then there must necessarily be a precise cause for the existence of each...... middle of paper. ....Spinoza insists, it is nevertheless possible that two substances can be distinguished by virtue of sharing an attribute and yet be distinct in nature by possessing an attribute not shared by the other. Thus, while substance A shares an attribute with substance B – namely, both share attribute C – the former differs in nature from the latter in that each possesses an attribute not contained by the other. If the nature of substance A is attribute C and attribute D, and if the nature of substance B is C and E, then it appears that the nature of each, although each shares an attribute in common, is fundamentally distinct. Thus, it seems that Spinoza's commitment to the thesis that no two substances share the same nature or attribute is an error, and I therefore conclude that it is possible for two substances to share an attribute while being different in nature..