One main topic of this paper concerns two unconscious presumptions inevitably tied to our (perceptual and intellectual) knowledge: the presumption of reality and the presumption of the essential ideality of things. The presumption of reality holds that our sensations are primarily appearances of actual reality (and not just appearances in our mind). Here, in accordance with the very idea of reality, an appearance has to be, with respect to its content, both public and objectively determined, in order to confi rm itself as truly an appearance of something actual and real. In that sense, our perceptual knowledge turns out to be essentially founded both on sensory information and on the idea of reality. The presumption of the essential ideality of things holds the possibility, impossibility and necessity of a thing essentially dependent on its idea, i. e. on whether the idea is internally consistent or inconsistent and (when it comes to the necessary) on whether its contradictory idea is internally consistent or inconsistent. In that sense, our (intellectual) knowledge of the possible, impossible and necessary turns out to be founded on the idea of contradiction. Another main topic of this paper, closely connected to the fi rst one, concerns the epistemological implications of the two opposite conceptions of idea that dominate the Western tradition of philosophy. Namely, if ideas are but representations in our mind, then subjectivism with respect to our knowledge (whether perceptual or intellectual) seems to be unavoidable. However, if ideas are the very essences of things, then our knowledge (whether perceptual, founded on the idea of reality, or intellectual, founded on the idea of contradiction) is objectively determined. Thus, what lies behind the two opposite conceptions of idea turns out to be the opposition between the subjectivist and objectivist conception of our mind and knowledge. In the closing section of the paper, the issue of the justifi cation of the two conceptions of our mind is briefl y addressed, within the frame of both neutral (Cartesian) and common sense (Reidian) approach to epistemic justifi cation. The conclusion is reached that within the neutral approach neither conception seems justifi ed, while within the common sense approach the objectivist conception proves fully justifi ed.