David Hume’s test of the Cartesian epistemological framework

This post follows upon this post.

Hume’s approach to philosophy was both more commonsensical than Descartes’ and less physicalist than Hobbes’. Much like Descartes’, Hume’s philosophy begins with questions of cognition rather than being. An Enquiry concerning Human Understanding was written primarily as a scrutiny of the operations of the mind. This endeavor places Hume within the context of traditional, Cartesian epistemology rather than that of the Metaphysical Realism which preceded it.

However, as an empiricist, Hume claimed that ideas were derived from sensation rather than pure intuition. This meant that Hume’s theory of knowledge resembled Hobbes’ rather than Descartes’ despite Hume’s defiance of Hobbes’ attempt to build his epistemological edifice upon the geometry and physics of motion. In fact, Hume does not try to build a system of knowledge at all as had Descartes and Hobbes.

Instead, Hume’s project was a test of their claim that an adequate theory of human knowledge could be modeled solely upon the deductive methods of mathematics. Hume did not share Descartes’ faith in the power of human understanding and was skeptical that man ever possessed what is commonly called ‘knowledge’ in the first place. Rather, Hume concluded that indispensable aspects of human cognition were driven by human habit and custom rather than rational procedures.

In order to test the veracity of the Cartesian epistemological framework, Hume accepted its premises and then pushed them to their limits. Hume acknowledged two sources of human knowledge, or kinds of reasoning: matters of fact and the relations of ideas. These corresponded roughly to Hobbes’ sensation and ratiocination, respectively. However, and more importantly, Hume explicitly defined matters of fact and relations of ideas in opposition to one another. Doing so allowed him to distinguish the kinds of statements that were justified within the Cartesian framework and those that weren’t.

Matters of fact were not merely the content of sensations but were additionally construed as contingent statements regarding the world. The opposite of these contingent statements would be true without resulting in logical contradiction or violating the meaning of the terms. For example, perhaps a cat is on the table, but it could also be the case that there is no cat on the table just now and no logical contradiction arises from that statement. Furthermore, matters of facts are the sorts of statements that require investigation of what the world is like in order to decide whether they are true or false.

By contrast, the relations of ideas were statements which possessed a formal structure similar to statements from the logical or mathematical disciplines and which were either intuitively or deductively certain. These include the statements of geometry, algebra, and arithmetic. They were discoverable by thought alone, requiring no reference to facts in the world. In other words, they were statements of pure reason.

In sum, Hume allowed both sensation and pure reason a place among legitimate sources of knowledge and put them into the same foundationalist structure of basic and derived non-basic beliefs as Descartes and Hobbes had. Moreover, he took the additional step to clarify what sorts of statements were appropriate and exclusive to sensation and pure reason. Effectively, in assuming the truth of the Cartesian epistemological framework, Hume strengthened and clarified the logical rigor of the framework prior to testing it.

Still, he thought he had identified an aspect of human knowing – a kind of inference between cause and effect – for which this framework could provide no account. If Hume was correct that these inferences about cause and effect lack epistemological justification, then the Cartesian project of modeling human knowing on the deductive methods of mathematics fails for both Rationalists and Empiricists.

Here is Hume’s argument.

All reasoning about matters of fact is reasoning about cause and effect. Further, all reasoning about cause and effect is simply reflection on the similarities noticed between natural objects. Human experience of natural objects induces the expectation that similar effects will flow from reasonably similar objects. But Hume cannot find among the kinds of reasoning allowed by the Cartesian framework one which provides a rational account of the expectation of certain effects from natural objects man has not yet experienced.

If a body of like colour and consistence with that bread, which we have formerly eaten, be presented to us, we make no scruple of repeating the experiment, and foresee, with certainty, like nourishment and support. Now this is a process of the mind or thought, of which I would willing know the foundation. […] The bread, which I formerly ate, nourished me; that is, a body of such sensible qualities was, at that time, endowed with such secret powers: But does it follow, that other bread must also nourish me at another time, and that like sensible qualities must always be attended with like secret powers? The consequence seems nowise necessary. At least, it must be acknowledged that there is here a consequence drawn by the mind; that there is a certain step taken; a process of thought, and an inference, which wants to be explained. (1)

Hume then states the structure of this sort of inferences in more precise terms:

There two propositions are far from being the same:

– I have found that such an object has always been attended with such an effect and,

– I foresee, that other objects, which are, in appearance, similar, will be attended with similar effects. (2)

Hume claims, then, that while the first proposition is a matter of fact founded on reasoning about the relations of cause and effect and had through experience, the second proposition is not. Further, the second proposition is also not the kind of statement that comes from reasoning about the relations of ideas. Yet propositions of this kind make up the very core of empirical science.

Hume acknowledges that we can know that certain experienced effects are caused by specific objects, but he denies that experience of one object justifies conclusions about another, as of yet unexperienced object, no matter how similar the two objects may be. The sort of inference required to justify the second proposition is a cognitive process that is not founded on either cause and effect reasoning about matters of fact nor on the intuitive or demonstrative reasoning concerning the relations of ideas. Ultimately, since the Cartesian epistemological framework cannot account for this cognitive process so basic to human knowing, Hume denies the framework is epistemological justified.

Notes:

1. David Hume, An Enquiry concerning Human Understanding, p. 35.
2. ibid., p. 35.