Many students have trouble understanding Spinoza’s Proposition 1.5 and exactly what it is he is trying to prove. Here is what Spinoza said:
In Nature there cannot be two or more substances having the same nature or attribute.
If there were two or more distinct substances, they would have to be distinguished from one another by a difference either •in their attributes or •in their states (by Prop. 4). If they are distinguished only by a difference in their attributes, then any given attribute can be possessed by only one of them. Suppose, then, that they are distinguished by a difference in their states. But a substance is prior in nature to its states (by Prop. 1), so we can set the states aside and consider the substance in itself; and then there is nothing left through which one substance can be conceived as distinguished from another, which by Prop. 4 amounts to saying that we don’t have two or more substances ·with a single attribute·, but only one.
I think how you have to read it is to add the unspoken premise that even two distinct objects that in every other way are identical in their attributes would still be distinguishable by their different locations in space (That was Thomas Aquinas’s argument of individuation) and thus they can’t really have exactly the same attributes. So in that vein, if someone said “I have two apples, they are exactly alike in every attribute even the attribute of their location” that is really a metaphysical nonsense statement because we would not be able to discern any difference between these supposedly different apples and the speaker would be postulating imaginary entities.
