Actually, concerning the nature of gravitational force Newton (in his rightfully famous Philosophiae Naturalis Principia Mathematica) we find the well-known phrase "hypotheses non fingo." However, it is so well known that the nuances and extent to which Newton elsewhere seems indeed to suggest more than he states in this passage that it is more important here to note what Newton wrote elsewhere and quite explicitly. Namely, in a letter to Richard Bentley Newton writes:
"Tis unconceivable that inanimate brute matter should (without the mediation of something else which is not material) operate upon & affect other matter without mutual contact; as it must if gravitation in the sense of Epicurus be essential & inherent in it. And this is one reason why I desired you would not ascribe {innate} gravity to me. That gravity should be innate inherent & {essential} to matter so that one body may act upon another at a distance through a vacuum without the mediation of any thing else by & through which their action or force {may} be conveyed from one to another is to me so great an absurdity that I beleive no man who has in philosophical matters any competent faculty of thinking can ever fall into it." (emphasis added)
In other words, perhaps the plainest, clearest and most extreme critic of the idea that Newtonian gravitation should be understood as a force or in general as mediating interactions between and among "bodies" in the manner described by Newtonian mechanics was...Isaac Newton.
Elsehwere he makes it fairly obvious that God is the "agent" he personally (it seems) appears to find necessary to explain what it is that his "gravitational laws" merely describe (i.e., the equations and geometric arguments one finds in Newton's work concerning mechanics do not postulate a force in the sense of an actual physical force mediating interactions or otherwise existing and acting upon matter; they provide a description via a "blackbox" that one can use in order to make quantitative predictions and models). Indeed, the vast majority of the founders of physics and early modern science more generally were interested in natural philosophy as a form of a sort of applied theism, which is how today we have that most important and ubiquitious principle governing the nature and structure of a vast and varied number of physical theories- the principle of least action (formulated originally by Maupertuis and extended and generalized principally by Euler and with the same idea: that a most supreme and perfect Creator would endow in his creation such properties that reflect his essential nature and therefore that the laws of natural philosophy/physics should, would and are in fact those which are optimal)
A central motivation Einstein had in his construction of a general theory of relativity was that of forever ridding modern physics of the vestiges of radical nonlocality leftover from Newtonian gravitation (even after being reformulated in terms of locally acting fields).
Hence, we can today do what Newton himself intended: understand Newtonian gravitation not in terms of an actually existing physical force but as a descriptive "blackbox" into which we can plug constants and out of which we will obtain predictions that are, in the main, more than accurate enough. However, Newtonian gravitation is an effective force, meaning that it is understood to be a descriptive approximation underlying something radically different (again, something akin to what Newton intended) much the same way that equations governing ensembles in statistical mechanics is understood or the use of the RG flow in condensed matter physics and many-body physics is. It is somewhat different from effective field theories in general, however, as we do not quantize it up to some arbitrary level of precision whereupon we extract predictions via renormalization and regularization as we do in EFTs more generically.
