Argument For Substance Monism
Recall that for Descartes all of reality is either the substance "Body" or the substance "Mind". Spinoza, using the same rational principles by which Descartes reached his conclusion reaches a different conclusion: that everything that exists is really just a manifestation of one substance; all particular bodies and minds are just modes of this one substance. In other words, everything that exists (particulars) are just instantiations of this one maaaaaagical substance. According to my secondary source book, Spinoza's argument is comprised of four sub-arguments (the references in parentheses refer to Spinoza's text):
1. No two substances can share an attribute (1p5).
2. It pertains to the nature of substance to exist (1p7).
3. God--which Spinoza previously defined as the substance with all the attributes--exist. This follows from (1p7).
C. Since God exists, and by (1p5) no two substances can share an attribute, no other substance besides God can exist.
Before we happily launch ourselves into the individual arguments I just want to point out what Spinoza meant by "God". Spinoza's God: Spinoza didn't mean some capricious anthropomorphic God of the Bible who isn't subject to the same laws that apply to all particular things; by "God" Spinoza meant "Nature". There is some debate about exactly what he meant but for our purposes we can merrily go about substituting the word "Nature" every time Spinoza uses "God".
The Principle of Sufficient Reason (PSR): This rejection of God as free from the constraints of the laws of the universe was all part of his idea that "everything has to play by the same rules"; that is to say, if we want to hypothesize an entity or class of things that is subject to a different set of rules from everything else, there has to be a rational argument to give a reason for which this should be true. If a sufficient reason cannot be provided, then we have no reason to assume that there is any difference between the two classes of things being discussed. The PSR is central to Spinoza's method so keep an eye out for it! Once again, the PSR in plain language is: If'n you can't provide a good reason for a distinction then we should not apply the distinction.
Step 1: We Can Have Lots of Fun!
The first step of the argument is to demonstrate that "in nature there cannot be two or more substances of the same nature of attribute"; in other words two or more substances can't share an attribute. Given Spinoza's rationalist method, to prove his claim he gives a rational argument for how two substances could be individuated (if it's possible). In logical terms, Spinoza uses a reductio argument: that is; he begins with trying to prove the opposite of his desired conclusion and shows that it leads to a logical contradiction; since the opposite hypothesis leads to a contradiction, its opposite (Spinoza's) must be true. Lets take a look-see:
Spinoza gets out his ax and axes, what would it take to prove that two substances are indeed different substances? I.e. What is required to explain a substances identity and non-identity? Obviously, the only way to distinguish between substance A and substance B is through some difference in their properties. We know that substances in the Cartesian Dualist model have both primary attributes and modes that allow us to identify the type of substance they are. E.g., If a substance has extension (attribute) or texture and smell (modes) we can infer that the substance is Body. So if we were to distinguish between two substances (in the metaphysical sense) we would have to refer to either the attributes or modes of the substances.
Vs Attributes
Lets try distinguishing 2 substances via attributes. Recall that the end goal for Spinoza is to show that 2 substances cannot share the same attribute. But suppose that two substances did share the same attribute; if this is the case and we are relying on attributes to distinguish the two substances Spinoza's argument will fail. E.g., substance A has the attribute X and so does substance B. To identify substance A I say "it's the one with attribute X" but to identify substance B I also say "it's the one with attribute X". How can they be the same but different? (In Thai: same, same but different) This is a contradiction. So via reductio, where 2 things share the same attribute we shouldn't say that the two things are of different substance rather they are of the same substance. Of course this still leaves open the possibility that 2 substances have different attributes to which we will return; nevertheless it seems quite clear that Spinoza is correct in that two substances cannot share an attribute and still be identified as distinct substances.
Vs Modes
Well, maybe appealing to attributes doesn't work to show that two substances can share attributes...what about appealing to modes? Certainly modes vary from thing to thing maybe they can help...Spinoza's argument against using modes to distinguish between 2 substances that share an attribute hinges on the notion of ontological priority. (Ontological refers to "being".) As I'm sure you recall from the previous Spinoza entry last month both Spinoza and Descartes propose that some things are conceptually and causally prior to others. Relevant to our discussion is that substance is conceptually prior to attributes--i.e., the concept of extension presupposes the concept of substance; and primary attributes are conceptually prior to modes--i.e., you can't have modes of texture, smell, colour without having the prior concept of extension.
Now that we have the notion of conceptual priority clear as mud we can apply it to Spinoza's attempt to prove that two substances cannot share an attribute, through appeal to modes. Suppose substance C and substance D both share the primary attribute of extension. We know that we can't appeal to extension to individuate the two as separate substances...we tried that a few paragraphs back to no avail; lets instead try pointing to the differences in C and D's modes. Are you pointing? Point harder! Remember that the principle of priority implies that all modes are explainable via the primary attribute through which they arise. We can ask, in virtue of what does C's colour, shape, and texture arise? From it's primary attribute--extension. And in the case of D, which also shares extension, we can ax the same, "where do D's modes come from?" Why from its extension of course...C has its modes because it has extension and D has its modes because its extended.
Doh! Now we are back to referring to both C and D via their primary attribute (extension); and as we saw a few paragraphs up, there is no way for us to assert that two substances are distinct if they have the same primary attribute. So, again since assuming two different substances that share the same attribute leads to a logical contradiction, we must assume the opposite to be true; that when 2 things share an primary attribute, they are the same substance. In rationalist language: there is no sufficient reason for which we should assume two things sharing the same attribute are two different substances.
Leibniz' Objection
So, if you understood any of that (it took me reading my book at least 3 times very very very sloooooooowly) the possible objection should be obvious. Like I said, only if you understand what's going on. If you do, then you are either a philosophy genius or I am a genius teacher...I'll make that an inclusive or. Ok, back to Leibniz' objection: Spinoza says a substance can have more than one attribute; indeed God has "absolutely infinitely many" of which extension and thought are only two. So why not suppose that there are 2 substances that share one attribute but not another. Eg., substance E is a rock, it only has extension. Substance F is a human, it has extension and thought. Why can't we just say "we can identify substance E and F as being distinct because E is the one that doesn't have thought and F is the one that has thought."
Because his works were published post-modem obviously he didn't address this objection directly but we can apply some of his principles and construct a good reply that Spinoza could have given. The principle required for this reply are that each attribute should, by itself, be able to pick out the substance which is conceptually prior to it. I'm going to let you ponder this cuz I needs me some sleep. If anyone actually read this entry all the way to the end, you win one internet!
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