Spinoza argues that if God exists, He is a single continuous substance that constitutes all of reality. How does this argument go, and how does Spinoza use it to support the conclusion that there is a match between ideas and physical objects?
God or Nature is the unique substance (Ethics,Ip14), existing essentially (Ip7), infinite in power (Ip8), and characterized by infinite attributes, each constituting its essence (Id4). Finite things are but modifications of that substance, and not themselves independent beings (Ip14); bodies in particular are modes of substance conceived under the attribute Extension. God or Nature, as the ultimate cause of all things, is also the cause of all the particular modifications of extended nature (Ip18).
Thought is another attribute through which substance is conceived (IIp1). Since substance is unique, and the attributes simply various essences under which it is conceived, the series of finite modal causes in each attribute must operates strictly in parallel with one another (IIp7). For every modification of Thought there is a modification of Extension it mirrors, and vice versa; and the causal order of the one is perfectly matched with that of the other. The mind, a finite mode of Thought, is, under this parallelism, simply the idea of the body to which it corresponds under the parallelism (IIp13). The economy of ideas is precisely as closed, necessitated and deterministic as that of bodies.
Comment on "infinity"
[Ethics, Def.2] A thing is said to be finite in its own kind [in suo genere finita] when it can be limited by another thing of the same nature. For example, a body is said to be finite because we can always conceive of another body greater than it. So, too, a thought is limited by another thought. But body is not limited by thought, nor thought by body.
[Def.6] By God I mean an absolutely infinite being, that is, substance consisting of infinite attributes, each of which expresses eternal and infinite essence.
Explication I say "absolutely infinite," not "infinite in its kind." For if a thing is only infinite in its kind, one may deny that it has infinite attributes. But if a thing is absolutely infinite, whatever expresses essence and does not involve any negation belongs to its essence.
According to Id2, infinite must mean "unlimited": we may think at lack of spatial or temporal limits.
The 'match between ideas and physical objects', points up a conundrum which every commentator on Spinoza's epistemology has wrestled with for over 200 years, some with more success than others. But it still remains more or less hidden from view. The match exists at the nexus in the mind where 'thought' and 'object' conjoin. He called it 'idea' and 'ideatum'. In Latin the difference and similarity between the two helps to clarify our English language confusion. The idea and its object are one and the same thing, expressed in two different milieus. An apple hangs from a tree and once someone views it, observes it, experiences it, it comes alive in that person's mind. That apple in the person's mind is not a concept, it is not a mental picture. It is the essence of that apple hanging on the tree reproducing itself spontaneously and automatically in the mind. Here's one more example:
Believing firmly that if we cannot explain even the greatest complexities in simple terms then that reveals that perhaps we do not really understand them ourselves; I constructed the one minute explanation of Spinoza's 'idea'. This version was for my massage therapist who I see weekly as part of a recovery regimen. I mentioned my Pamphlet number one, on Baruch's TIE, which I'd 'gifted' her. I told her not to worry about reading the pamphlet until she retired, but that if she wished, that I would tell her right on the spot, the gist of the whole book; to understand Spinoza's 'idea'. And here it is;
I told her that in her mind she had a file, called 'Charles' body. Each time she worked om me she automatically 'pulled' out the file and used it to remind herself what areas she'd worked on last week and this gave her a platform for today's work. As she worked on me she would compare with her mind's recollection from last week, what she needed to accomplish today. Once she finished my therapy, the file was automatically returned to her brain to be retrieved only when she needed it next week. This 'file', I told her, is real, it's cumulative it's a singularity, it's one of many more of these files which she retains in her brain and which her mind only retrieves when she needs them. Each of these 'files' is one of Spinoza's 'ideas'. A brain can store these but only the mind can retrieve, update and restore them back in the brain. That's it, that is one way, and only one way to describe as simply as possible what Baruch meant. Sapere aude, Charles M. Saunders
The work done on my body is the 'object'. The file in her mind is the 'idea'.
For more on this read Spinoza's "On the Improvement of the Understanding". It is only 43 pages in length but contains a non-geometric explanation of the 'idea'. This document holds the key to unlocking the 'clear labyrinth' of the Ethics. CS @Keelon