It is only by passing through the naïve natural belief that the imagination fabricates this "arbitrarily invent[ed]" philosophical system. Hume ends by voicing strong doubts about any system based on "such trivial qualities of the fancy", and recommending "[c]arelessness and in-attention" as the only remedy for skepticism. Next, Hume presents a brief critique of "antient philosophy" (traditional Aristotelianism) and "modern philosophy" (post-Scientific Revolution mechanical philosophy), focusing on their rival conceptions of external objects. As for the incomprehensible "fictions of the antient philosophy", he thinks they can shed further light on human psychology. We begin with contradictions in "our ideas of bodies": between seeing bodies as ever-changing bundles of distinct qualities, and seeing bodies as simple unities that retain their identity across time. We reconcile these contradictions by fabricating "something unknown and invisible" that underlies change and unifies the distinct qualities together: i.e., the "substance" of traditional metaphysics. Similar fictions, fabricated by the imagination to resolve similar difficulties, include "substantial forms", "accidents", and "occult qualities", all meaningless jargon used only to hide our ignorance. Modern philosophy, however, claims to disown the "trivial propensit[ies] of the imagination" and follow only solid reason (or, for Hume, "the solid, permanent, and consistent principles of the imagination"). Its "fundamental principle" is that secondary qualities ("colours, sounds, tastes, smells, heat and cold") are "nothing but impressions in the mind", as opposed to the primary qualities ("motion, extension, and solidity") that exist in reality. But Hume argues that primary qualities cannot be conceived apart from the secondary qualities. Thus if we follow solid reason and exclude the latter, we will be forced to contradict our own senses by excluding the former as well, thereby denying the entire external world. Hume then examines "the nature of the mind", starting with the materialist-dualist debate over the substance of the mind. He rejects the whole question as "unintelligible", for we have no impression (and therefore no idea) of any substance, and defining 'substance' as "something which may exist by itself" doesn't help (each of our perceptions, Hume argues, would then count as a distinct substance). Turning to the question of the ""local conjunction"" of mind and matter, he considers and endorses the anti-materialist argument which asks how unextended thoughts and feelings could possibly be conjoined at some location to an extended substance like a body.