Due to the amount of discussion generated—both in agreement and dissension—from the article entitled “10 Problems with Spinoza’s God,” I have decided a more in-depth and explicit version of it is in order, so as to better respond to critics, as well as to offer more to chew on for my article’s proponents. If you have yet to read the original article, it is linked here, and beneath it also is linked a translation of Spinoza’s original text, as well as the Stanford Encyclopedia of Philosophy’s summary of it. Don’t worry if you’re coming into the topic without any background knowledge of Spinoza’s work whatsoever, the Stanford summary of Spinoza’s metaphysics should be more than enough to get you going (and hey, unless you plan on debating a pantheist anytime soon, you won’t need to be too well versed in Spinoza anyway)! And if you’re too unfamiliar with some of the vocabulary of Thomism and Scholasticism, feel free to look up an unfamiliar word you come across here in the Dictionary of Scholastic Terminology.
Now, to readdress Definition VI of Spinoza’s argument for monism, Spinoza holds that by God he means “a being absolutely infinite—that is, a substance consisting in infinite attributes, of which each expresses eternal and infinite essentiality,” with his explanation being, “that which is absolutely infinite, contains in its essence whatever expresses reality, and involves no negation” . Thus, by defining God as such, he inadvertently expresses his conclusion, or Proposition 14, which states that “besides God no substance can be granted or conceived” , due not to his own proofs, but due to the necessarily implied consequence of his conclusion, namely, that that which caused the universe is indistinguishable from it. Now, Aquinas tells us that that which caused the universe “all men speak of as God” , Spinoza being no exception to this, since he too supposes the cause of the universe to be God, though he defines Him differently from Aquinas. As that which causes the universe necessarily exists , and this all men call God—Spinoza being no exception—then, necessarily, that which he calls God must exist, and since that which he calls God “contains in its essence whatever expresses reality [ie everything], and involves no negation,” being, then, by virtue of it its essence, all that is, it is evident that, upon such considerations, Definition VI necessarily implies Proposition 14, that “besides God no substance can be granted or conceived” , making, then, Spinoza’s argument circular, or at least backwards, to begin with, as it has been shown that the definition implies the proposition.
Moving now to Axiom IV, Spinoza claims that “the knowledge of an effect depends on and involves the knowledge of a cause” . As to grasp the essence of a thing is to have a perfect knowledge of it, and as Spinoza defines the essence of any given thing as to be extended and thoughtful [5, 6], adopting and adapting the notion from Descartes’s dualism , it follows that in order to know that something is extended and thoughtful, it would be too required to know that that which caused it is extended and thoughtful, or, as St Thomas would say, that it has corporeal quantity  and at least a sensitive soul . Yet, many who believe that earth is extended and thoughtful, or has corporeal quantity and at least a sensitive soul (eg Jains), do not believe that it has a cause (materially or efficiently) which too possesses extension and thought. Therefore, according to Spinoza’s own understanding of essence,* unless those who believe that earth is extended and thoughtful, without believing it to have an extended and thoughtful cause, do not believe that the earth is extended and thoughtful (which is a logical contradiction), his Axiom IV is false.
Continuing on, Axiom VII of Spinoza’s argument, which states “If a thing can be conceived as non-existing, its essence does not involve existence” , inadvertently points to a massive flaw of Spinoza’s argument. As Spinoza is arguing for the validity of monism, all which exists must necessarily have an essence which involves existence, or as Aquinas would say (yes, I’m referencing Aquinas again—this is a Catholic website, after all!), has an essence identical with its being, because such is required of God , and Spinoza admits such in Proposition 7 . However, it is apparent to the senses that any given thing with potency has the ability to cease existing—a rock, for example, can be destroyed, and hence cease to exist. Thus, anything which has potency does not have an essence which involves existence. Though some may object that Spinoza’s notion of modes solves this, the concept of modes is reliant on the validity of monism for itself to have weight; hence, as such presupposes monism, it can not be used to defend monism. This means, of course, that unless monism can be proven, which can not be done without the modes being used, which can not themselves be valid without monism having been proven, the apparent ability of objects with potency to cease existing is an insurmountable obstacle to the argument Spinoza presents for monism, because to defend this proposition from the objection on the grounds of the modes is to presuppose the conclusion of the argument.
Now reaching his propositions (Hooray! We made it to his actual reasoning!), the second is patently false. Spinoza proposes that “two substances, whose attributes are different, have nothing in common” . To justify this, he recalls his third definition, which states “by substance, I mean that which is in itself, and is conceived through itself: in other words, that of which a conception can be formed independently of any other conception” . As Spinoza considers the attributes if a thing “that which the intellect perceives as constituting the essence of substance” , and that the only true attribute of any given physical thing is that of extension (at least, at this point in his argument) [2, 5, 6], all things which have the attribute of extension must have an identical substance. However, here Spinoza is merely confounding accident with substance, in that the accidental quantity of the matter of a thing does not determine that which a thing is (and hence nor does the fact that it possesses material quantity at all—ie extension. Spinoza inherited this error from Descartes). Hence, that which is an attribute of the accidental matter—in this case, extent—is not constituting to the substance, especially under the consideration that substance is by definition the immaterial form of a thing, making nothing substantially extended, but only materially extended (for form itself has no extent). Therefore, it is not the case that being extended is the essence of anything, nor is it the case that to conceive of anything which is extended is to conceive of the essence of all extended things. Therefore, while two substances may have the accidental attribute of extension, they are not necessarily the same substance, because their essence is not to be extended, and (moreover) it appears to the senses that their beings are distinct, one being able to cease existing while the other continues to (ref the above paragraph).
As to the Rest…
Beyond this point the further issues noted in the original article need no further explanation, nor would they likely benefit from it. Therefore it is at this point that I make my leave of the topic, although I may return to it at a latter point in time, arguing against monism/pantheism via the attributes shown to be true of God by way of more explicitly Thomistic methods. Why Thomistic methods? Well, because I’m a Thomist, of course!
*Spinoza’s understanding of essence is itself suggestive of monism, in that if all physical things have an identical essence which is to be extended, they are all then the same thing. However, this notion too is addressed in paragraph four.
 Spinoza. Eth I. D6. trans. Elwes
 Spinoza. Eth I. P14. trans. Elwes
 Aquinas. ST I. Q2. A3. trans. Fr’s OP
 Spinoza. Eth I. A4. trans. Elwes
 Spinoza. Eth II. P1–2. trans. Elwes
 Spinoza. Eth I. D4. trans. Elwes
 Descartes. Med II. trans. Veitch
 Aquinas. ST I. Q3. A1. trans. Fr’s OP
 Spinoza. Eth I. A7. trans. Elwes
 Aquinas. ST I. Q3. A4. trans. Fr’s OP
 Spinoza. Eth I. P7. trans. Elwes
 Spinoza. Eth I. P2. trans. Elwes
 Spinoza. Eth I. D3. trans. Elwes
 Spinoza. Eth I. P3. trans. Elwes
 Spinoza. Eth I. A5. trans. Elwes
 Spinoza. Eth I. P5. trans. Elwes
 Spinoza. Eth I. P4. trans. Elwes
 Spinoza. Eth I. P6. trans. Elwes
 Aquinas. ST I. Q3. A5. trans. Fr’s OP