  The Phaedo's discussion of Recollection suggests that there is something inherently flawed with empiricist or abstractionist accounts, at least those that attempt to derive any concept from our contacts, perceptual or linguistic, with the external world. It seems that Plato thinks that the deficiency of the external, sensible, material world vitiates all efforts to build or acquire concepts from it. The deficiency of the sensible material world makes it an unreliable source of information. Depending on how one accounts for this deficiency, the trouble for perception, and belief based on perception, is explained in different ways. Aristotle's account emphasizes that it is changing. Another source of difficulty is that the abstract, general Forms are always manifested in a concrete, determinate fashion (See §7, supra). The Affinity Argument points to the complexity of material particulars. Rather than present a single property, as it were, to perception, perception is required to focus on some aspect of the complex particular. If it is also true that at least with respect to some properties, namely the incomplete or relative properties addressed by the Compresence accounts, every object is both F and not-F, then no sensible will be an unqualified bearer of these properties. Perception, considered in its own right, seems to be unable to explain how any feature of an object is selected for study. It also seems that Plato thinks that the psychological faculties of perception, or even belief, are incapable of processing the information in a reliable manner, or at least in a manner requisite for knowledge. At times it seems that they distort or alter perceptions (Phaedo, 65ff). At other times, e.g., when we emphasize the difference between the faculties, it seems that they are confined to work on whatever they process in such a manner that they do not pass along anything to the rational faculty, or if they do, they pass it along to belief. One problem is that whenever one engages in perception, and belief based on perceptual reports, one can never overcome the inherently perspectival situation. For instance, no matter where one is situated, the round penny will appear to the eye, we might say, somewhat elliptical. Or, to use Plato's example from Book Ten, the straight stick in water will seem bent (owing to the laws of refraction), even though an experienced person will believe that it is straight  (602c-603a).[24]