Can a rationalist ever be a materialist?

Question

Note that I am using rationalist in its strict philosophical sense, as in rationalist like Descartes or Leibniz, not rationalist as it might be understood colloquially. from the Encyclopedia Britannica:

Rationalism, in Western philosophy, the view that regards reason as the chief source and test of knowledge. Holding that reality itself has an inherently logical structure, the rationalist asserts that a class of truths exists that the intellect can grasp directly. [....] Rationalism has long been the rival of empiricism, the doctrine that all knowledge comes from, and must be tested by, sense experience. As against this doctrine, rationalism holds reason to be a faculty that can lay hold of truths beyond the reach of sense perception, both in certainty and generality.

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In this question, the OP wonders whether materialism and empiricism are the same or not. It is pointed out in the answers that empiricism and materialism are not the same at all, one being an epistemic method, while the other is an ontological position, and that they shouldn't be conflated.

Although I understand that the two are not the same, I feel that the OP is right in sensing a relation between the two, for the following reasons:

• Although not all empiricists are materialists, those that are materialist are so based on an empiricist position of believing only in that which can be observed.
• Insofar as empiricism's antithesis is rationalism (See the SEP article), one can say that while empiricism doesn't necessarily imply materialism, its antithesis, rationalism, does imply some form of idealism or at least dualism, for how else would there be truths that are directly knowable by the mind, regardless of the senses?

My questions:

• Is it possible to be both a rationalist and a materialist? How can one reconcile materialism with the idea (see what I did there :-)) that there are truths which are independent of sense data?
• Have there been any famous rationalist materialists?

Answer 1

Spinoza, Descartes's and Leibniz's contemporary, has often been identified as a rationalist materialist, at least historically, modern scholarship is more skeptical of the label, see Zammito's Genesis of Kant's Critique of Judgment. His explanation of the access to reality beyond senses is the prototype of such an explanation for materialists, idealists and dualists alike, an extra "sense", another mental faculty, "intellectual intuition" which provides "the third kind of knowledge". Spinoza's version of intuition is quite a bit more subtle than Descartes's and Leibniz's, whose was purely conceptual. It is what Kant will later call "intellectus archetypus" in Critique of Judgement, an ability to perceive reality in "wholes" rather than discursively, an ability inherent in all matter, according to Spinoza, one of its attributes. Spinoza considers causality and logical inference as just two different perspectives of the same "substance", identifiable as both God and matter:

“Thought is an attribute of God, or God is a thinking thing. Extension is an attribute of God, or God is an extended thing... whether we conceive nature under the attribute of Extension, or under the attribute of Thought, or under any other attribute, we shall find one and the same order, or one and the same connection of causes, i.e., that the same things follow one another... We may add that our mind, in so far as it perceives things truly, is part of the infinite intellect of God; therefore, the clear and distinct ideas of the mind are as necessarily true as the ideas of God... The human Mind has an adequate knowledge of God's eternal and infinite essence [sic!]”.

While "intellectual intuition" was made famous by idealists, like Fichte and Schelling, "ideal perception" is not exclusive to them. Husserl produced a demystified version of it under the name of "categorial intuition", although he was an agnostic rather than a materialist, and "independent of sense data" is not quite how he'd put it. Rather, perception fuses both sensual and conceptual aspects, and acts of sense perception are similar in nature to intuitive acts of ideation, that are built on them in reflection and yield ideal content. "Sense data" is a theoretical fiction. Taken empirically, ideation theory is supported by developmental and cognitive psychology, and was embraced by analytic mathematical realists who are not Platonists, like Gödel, see in On Husserl's Mathematical Apprenticeship, and Australian school of Aristotelians. Penelope Maddy gives a materialistic version of "categorial intuition", inspired by Gödel's remarks, in Perception and Mathematical Intuition:

"Presumably, the faculty of mathematical intuition itself is "closely related" to this new sort of relationship between ourselves and reality which is responsible for the presence of these abstract elements in our perceptual experience... sense-data theories according to which it is not even possible, strictly speaking, to perceive physical objects... seem to me false... What I want to suggest now is simply that we do acquire perceptual beliefs about sets of physical objects, and that our ability to do this develops in much the same way as that in which our ability to perceive physical objects develops...".

Another modern example is Marjorie Grene, a prominent philosopher of biology, who reinterpreted Merleau-Ponty’s perceptual phenomenology in a similar vein.