We know him only by his most wise and excellent contrivances of things, and final causes; we admire him for his perfections; but we reverence and adore him on account of his dominion. For we adore him as his servants; and a God without dominion, providence, and final causes, is nothing else but Fate and Nature....All that diversity of natural things which we find, suited to different times and places, could arise from nothing but the ideas and will of a Being necessarily existing.--Isaac Newton "General Scholium (2nd edition of Principia) [Motte Translation].
Newton clearly embraces God's providence and general final causes. I write 'general' final causes because in the body of the Principia, particular or local final causes are absent in his argument. (In that limited respect, Newton accept Descartes's strictures on physics.) In the final sentence of the quoted passage, Newton offers a kind of design argument (from design to design). The underlying argument of this design argument is inductive: the (apparent) functionality of natural things must be ascribed to the mind and volition of a designer. Let’s ignore, for the sake of argument, the epistemic status of such arguments from apparent design to designer. It is worth noting that Newton does not explain why this designing-Being must be “necessarily existing.” Even if one were to grant the argument that goes from apparent functionality of natural things at all times and places to the existence of a designer, the modal status of such a designer – that s/he is necessarily existing – is not evident. Whatever else we can say, there are suppressed premises about the nature of modality here.
Newton, too, must have felt some weakness in his position because he added a sentence to the third edition of the General Scholium. The paragraph now reads:
We know him only by his most wise and excellent contrivances of things, and final causes; we admire him for his perfections; but we reverence and adore him on account of his dominion. For we adore him as his servants; and a God without dominion, providence, and final causes, is nothing else but Fate and Nature. Blind metaphysical necessity, which is certainly the same always and every where, could produce no variety of things. All that diversity of natural things which we find, suited to different times and places, could arise from nothing but the ideas and will of a Being necessarily existing. [Emphasis added]
A1: (Metaphysical) Necessity implies homogeneity and homogeneity implies (metaphysical) Necessity
A2: Homogeneity and variety are disjunctive alternatives (suppressed)
P1: We observe a particular kind of variety
C: Therefore, no metaphysical necessity.
C is associated with Spinozism. Here, too, we find a puzzling relationship between necessity and observed reality. We can understood what's going on in these arguments by looking at Clarke's writings. For, in his correspondence with (a then very youthful) Joseph Butler, Clarke explains a key assumption A1 in the homogeneity argument (recall). He writes: “Necessity absolute and antecedent in the order of nature to the existence of any subject has nothing to limit it; but if it operates at all (as must needs do), it must operate (if I may so speak) everywhere and at all times alike….” (Clarke’s “Answer to Butler’s Third Letter,” Dec 10, 1713)
So, what we have here is a view about metaphysical modality. To put Clarke’s point informally: absolute necessity has the same impact everywhere and all times and should have the same consequence everywhere and all the time. So, more precisely, I call metaphysical modality the idea that if such ‘necessity’ operates in some respect, Y, then we ought to expect Y to be homogeneously universal in relevant ways. And if the most fundamental form of necessity operates in all (possible) respects, then we ought to expect general homogeneity (as in A1 in the homogeneity argument).
We can explain the intuition behind Clarke's idea as follows: if some property or entity Z is present everywhere and at all times then this cries out for an explanation. An appeal to miracles is not allowed. (Clarke really likes the PSR.) So whatever the ultimate principle or cause of Z is, it must be of the right sort to have the same effect everywhere and at all times. Now, to be sure, the list of things that are universal in this way is going to be quite short; it contains, space, time, the truths of arithmetic and, perhaps (post Newton), gravity. (The list may also include God if s/he is immanent in nature--as it is for Newton.) Once laws are conceived as being universal in reach, they, too require some such underlying principle. According to Clarke necessity is the only kind of cause/principle that can produce qualities or entities that have some such universal reach.
To see how far reaching this view of necessity is according to Clarke, note that he asserts that necessity is the “formal cause” of God (Clarke’s “answer to sixth Letter to another Gentleman” [published first in 1738]). That is to say, Clarke does not just allow the question, ‘what causes God?’ to be intelligible, but it even has a proper response: one can say that for Clarke God is in a certain sense, the consequence of a species of necessity. (Clarke also uses this view of necessity to rule out multiple Gods.) Clarke, thus, flirts with aspects of Spinozism as Leibniz accurately discerned. With that in mind we can turn back to Newton.
There is, in fact, an unmistakable (but generally overlooked) other echo of this doctrine of metaphysical modality in the relationship between necessity and God in the General Scholium: “’Tis allowed by all that the supreme God exists necessarily; and by the same necessity he exists always and every where” (emphasis added). In the quote, Newton insists (i) that God’s necessary existence is common ground among all philosophers. Newton, then, (ii) ascribes to necessity explanatory ‘power’ to help account for (at least some of) God’s properties. It’s this type of explanatory power of metaphysical modality that is captured by Clarke’s claim that necessity is a formal cause.
In the quoted design to design argument of the General Scholium of the second edition of the Principia, Newton’s seems to rely on the idea that if some effect Y is universal temporally and spatially, then we can infer or posit as the ultimate cause something that itself is necessary in a way that accounts for Y. So, given that we find (let's grant for the sake of argument) evidence of functionality (or design) everywhere and at all times, we need for Newton to posit a necessary being with designer qualities. [He rules out chance on other grounds.] Newton has other arguments for this claim (including features of the homogeneity argument I have not articulated in this post--P1 figures into such arguments). Whether the ultimate necessary cause so posited acts by way of intermediary (or secondary) causes we can leave un-examined here. Conversely, and more boldly, if some purported feature is not present everywhere and at all times in nature [for example, a dense plenum without diversity], we can also say make claims about the character of necessity (as in C): the kind of thick necessity postulated in A1 (and the one -- mistakenly -- attributed to Spinozism) does not obtain in the universe because we observe very specific forms of variety. For Clarke and Newton, that's compatible with two further facts: one (as is well known) with a God who could have arranged some things otherwise; second, with metaphysical modality being a valid principle that explains all that is universal in the universe.