In the Private Thoughts (1619–22), Descartes stipulates that “man has knowledge of natural things only through their similarity [per similitudinem] to the things which come under the senses. Indeed, our estimate of how much truth a person has achieved in his philosophizing will increase the more he has been able to propose some similarity between what he is investigating and the things known by the senses” (AT X 218–19, CSM I 5; modified). By this measure, Descartes became a very accomplished philosopher, for in his published and unpublished work he frequently uses analogies between what comes “under the senses” and “natural things.” Specifically, Descartes’ analogies identify similarities between the effects of observable phenomena and processes, whose causes we know, and natural effects, whose causes we do not know. By way of his analogies, Descartes discovers or makes plausible the existence of specific unobservable natural causes and thereby provides a causal explanation in physics.

Although analogical reasoning is just one component of Descartes’ scientific method, which may be characterized as a version of hypothetical deduction minus careful confirmation (see Clarke 1982, Sakelleriadis 1982, and McMullin 2008 and 2009), it is with his analogies that Descartes bridges the gap between the world of experience and the moving and colliding particles at the microscopic level that ultimately explain the effects we observe. Yet Descartes rarely used the Latin term from which our “analogy” derives, let alone its French cognate analogie (cf. AT XI 158; see Galison 1984). The analogies from the Dioptrics (1637) that I discuss here are “comparisons” (comparaisons), and those from the Meteors (1637) are instances of reasoning by “example and similarity” (exemplum & similitudinem) (AT VI 83 and I 422, respectively). In the Principles (1644), analogies are typically “comparisons” (comparationes) or efforts to “compare” (consero/comparo) (e.g., AT VIIIA 87 and 110, respectively). In the famous Rule 8 from the Rules for the Direction of the Mind (1620s), Descartes’ method does not involve seeking “analogies” but advises us to “enumerate all the other natural powers so that, by means of knowledge of some other one, [we] might come to understand [the action of light]…, at least by imitation [imitationem]” (AT X 395, CSM I 29).