[Below is a draft of section 3B of a handbook article on "Spinoza and Science" forthcoming in OUP Handbook on Spinoza, ed. M. Della Rocca. I offer what I take to be a new reading of Spinoza's "Common Notions" and how we should not read them as akin to laws of motion. Comments welcome!--ES]
3B Common Notions (and laws of motion/thought)
Recall that in the TTP, Spinoza writes that “in examining natural things we strive, before all else, to investigate the things which are most universal and common to the whole of nature- viz., motion and rest, and their laws and rules, which nature always observes and through which it continuously acts and from these we proceed gradually to other less universal things,” (TTP 7.27; III/102). Spinoza alludes here to an important concept in his epistemology: so called “common notions.” In a handwritten note that Spinoza added to TTP he makes clear that common notions are stepping stones to adequate knowledge of God; in particular that God “exists necessarily, and is everywhere” and that God’s nature is presupposed in all things we conceive. (III/252-253)[1] So, while in this chapter I have been emphasizing a skeptical strain in Spinoza this concept seems to offer a robust route to adequate knowledge of the second kind (E2p40S2). In particular, one might think that I have given far too much attention to the unattainability of the third kind of knowledge and the limitations of the first kind of knowledge within Spinoza, while downplaying the presence of the second kind. Yet, Spinoza writes that human minds contain adequate ideas (E3p1dem) and refers back to E2p40S2; so common notions seem available to all.
The fact that “rules” of motion of rest are common notions is more important. This does echo a Cartesian program of scientific explanation with laws of motion and/or rules of collision. But the similarity with Descartes is superficial, as reflection on the nature of common notions reveals: common notions are structural features that all modes within an attribute share (E2P38, the corollary appeals to Lemma2, which in turns follows from the definition of a body E2D1). So, just as there are common notions of modes of extension, so there must be common notions of modes of thought. To put the point metaphorically, the economy of thought is just as rule-governed as the economy of nature for Spinoza (a most un-Cartesian thought). Of course, given parallelism (and 2p39, more explicitly), this means that these laws and rules, whatever their content, are going to have a high degree of generality and relatively little specificity. So, what are common notions?[3]
First, common notions are about qualitative not quantitative properties of extension. The manner or magnitude of such properties is extrinsic, and, thus, not a common notion. This becomes clear by reflection on how Spinoza characterizes common notions: common notions are qualities that all bodies share regardless of their state (see, especially, E2p38-39). Second, these properties do not just have a high degree of generality—they are common to all bodies (E2L2, cited in 2Ep28C)--, but the manner in which they are present within each and all bodies is also equal (E2p39Dem). The best way to make sense of common notions is, thus, to suggest that they are intrinsic properties of modes within an attribute (in Spinozistic terms they share an “affection”) and that they reflect the peculiar modal qualities of such a mode: for example, all bodies are equally capable of motion and of rest, of moving slower and quicker (E2L2), capable of being an efficient cause, of co-determining / terminating other bodies (1ED2, 1Ep28, E2L3), etc.
This last feature certainly draws Spinoza very close to Descartes’ laws of motion. For, example, Descartes’ first law states, “that each thing, as far as is in its power, always remains in the same state; and that consequently, when it is once moved, it always continues to move” (Pr II 37) and this fairly close to Spinoza’s corollary to 2EL3: “a body in motion moves until it is determined by another body to rest; and that a body at rest also remains at rest until it is determined to motion by another.” It is fair to say that Spinoza makes explicit what Descartes intended, that bodies are causes of each other’s motion and rest. So, Spinoza is Cartesian in so far that he accepts Descartes’ general program by which observed changes in motion (or rest) encourage the search for other bodies that caused these changes. (I qualify this in next paragraph.)
Even so, there are interesting differences: Cartesian ‘inertial’ motion is a consequence of the state-preserving power inherent in each thing, while Spinoza offers no such consequence relation in his Lemma.[4] A more important difference is that Spinoza lacks the equivalent of Descartes’ second law of motion: “all movement is, of itself, along straight lines” (Pr II 39). This is no trivial matter. It means that Spinozistic ‘inertial’ motion can take any ‘shape’ (circular, rotational, zigzagging, etc.) Intuitively Spinoza’s move makes sense: from the point of view of (say) eternity it is not obvious why states (of motion) need to be preserved along a straight line. This requirement seems to introduce an arbitrary directionality and even geometry into mode continuation/preservation. Given that Spinozistic laws of extension and laws of thought are, in some important sense, the same, such directionality would probably make a mockery of the very possibility of finding rules of thought that are identical to rules of extension (and any other attribute). It is also by no means obvious how the directionality requirement can be derived or justified metaphysically.[5] The downside of Spinoza’s approach is that it is very hard to see how in the absence of a detectable body (say) B acting as cause(s) on some body (A) we can ever say about some moving body (A) that it was in ‘inertial’ motion or not. Given that E2A1” explicitly allows that the way in which bodies move each other (as causes) is potentially heterogeneous the epistemic complications of using Spinoza’s Axioms and Laws as foundations for a science of motion are only increased. So, commentators that attribute to Spinoza the idea that his common notions enter into his science of motion saddle Spinoza with a decidedly unpromising physical science.
Now it is possible that Spinoza did not recognize any of the problems I have indicated. (Note, by the way, that I am not relying on later developments in physics.) It is possible, of course, that even after Christian Huygens published his masterpiece, Horologium oscillatorium sive de motu pendularium (1673), which articulated how Galilean principles could be developed into a science of motion, Spinoza was unwilling to drop his alternative approach. But given that Spinoza has so many criticisms of mathematical physics, a more obvious interpretation presents itself. Spinozistic common notions are not the foundation of a Spinozistic physical science (analogous to Cartesian / Huygensian / Leibnizian / Newtonian) mechanics; rather they capture secure knowledge of the modal qualities that are intrinsic to all modes of an attribute. This is the meaning of E5p4: “There is no affection of the body of which we cannot form a clear and distinct concept.”[6] That is to say, common notions provide us knowledge of the nature of bodies (E2P16). This is not nothing, of course, and such common notions are significant because with Spinozistic metaphysics they provide hope that access to third kind of knowledge is available to mere mortals (E2p47S).
NOTES
[1] See E.M. Curley, (1990) “Notes on a Neglected Masterpiece (II): `The Theological-Political Treatise' as A Prolegomenon to the Ethics,” Central themes in early modern philosophy: essays presented to Jonathan Bennett edited by Jan A. Cover and Mark Kulstad, Indianapolis, Hackett Publishing, 118-119.
[2] See, e.g. Curley 1990: 119.
[3] In the secondary literature one often finds the answer: “infinite modes” (and these are often thought to be scientific laws of nature). The evidence for this claim is remarkably thin (it requires reading E1p21-23 in light of letter 83). But even if one grants the equation among “infinite modes,” “common notions,” and “laws of nature,” this does not license the further inference that common notions are the building blocks of a science of motion, or mechanics.
[4]When Spinoza does state his Conatus doctrine later at E3p6-7 it is traced back to Spinoza’s understanding of the expression doctrine (1Ep25C), God’s power (E1p34), what it means to be an “essence” (E1p36) and a “determinate nature” (E1p29). Motion is strikingly absent in motivating or explaining the conatus doctrine.
[5] This is, I think, why E2A2”, which does offer non-trivial directionality constraints on the way collisions proceed is offered as a further Axiom. It is very hard to see what justifies treating it as a common notion. It is also very different from Descartes’ third law of motion, which is supposed to govern collision (and from which the particular rules of collision are claimed to be derived).
[6] The demonstration of E5p4 reads as follows, “Those things which are common to all can only be conceived adequately (by IIP38), and so (by IIP12 and L2) there is no affection of the body of which we cannot form some clear and distinct concept.” The inference makes perfect sense if common notions pick out the knowable modal qualities that are intrinsic to all modes of an attribute; if extrinsic qualities are thought to be included in an “affection of the body” then Spinoza’s inference begs the question. This interpretation fits how Spinoza implies that the non-affections of the human body are not known to the mind at E2p24.
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