The New Academy,[156] then, taught with Plato, that all things in their own nature were fixed and determinate; but that, through the constitution of the human mind, it was impossible _for us_ to see them in their simple and eternal forms, to separate appearance from reality, truth from falsehood.[157] For the conception we form of any object is altogether derived from and depends on the sensation, the impression, it produces on our own minds ([Greek: pathos energeias, phantasia]). Reason does but deduce from premisses ultimately supplied by sensation. Our only communication, then, with actual existences being through the medium of our own impressions, we have no means of ascertaining the correspondence of the things themselves with the ideas we entertain of them; and therefore can in no case be certain of the truthfulness of our senses. Of their fallibility, however, we may easily assure ourselves; for in cases in which they are detected contradicting each other, all cannot be correct reporters of the object with which they profess to acquaint us. Food, which is the same as far as _sight_ and _touch_ are concerned, _tastes_ differently to different individuals; fire, which is the same to the _eye_, communicates a sensation of _pain_ at one time, of _pleasure_ at another; the oar _appears_ crooked in the water, while the _touch_ assures us it is as straight as before it was immersed.[158] Again, in dreams, in intoxication, in madness, impressions are made upon the mind, vivid enough to incite to reflection and action, yet utterly at variance with those produced by the same objects when we are awake, or sober, or in possession of our reason.[159] It appears, then, that we cannot prove that our senses are _ever_ faithful to the things they profess to report about; but we do know they _often_ produce erroneous impressions of them. Here then is room for endless doubt; for why may they not deceive us in cases in which we cannot detect the deception? It is certain they _often_ act irregularly; is there any consistency _at all_ in their operations, any law to which these varieties may be referred? It is undeniable that an object often varies in the impression which it makes upon the mind, while, on the other hand, the same impression may arise from different objects. What limit is to be assigned to this disorder? is there any sensation strong enough to _assure_ us of the presence of the object which it seems to intimate, any such as to preclude the possibility of deception? If, when we look into a mirror, our minds are impressed with the appearance of trees, fields, and houses, which are unreal, how can we ascertain beyond all doubt whether the scene we directly look upon has any more substantial existence than the former?[160]