37. Now consider the other side. One and the same object will necessarily present very different aspects under different subjective conditions, since it is _these_ which determine the aspect. The object cannot be to Sight what it is to Hearing, to Touch what it is to Smell. The vibrations of a tuning-fork are seen as movements, heard as sounds. In current language the vibrations are said to cause the sounds. Misled by this, philosophers puzzle themselves as to how a material process (vibration) can be transformed into a mental process (sensation), how such a cause can have so utterly different an effect. But I have formerly[209] argued at some length that there is no transformation or causation of the kind supposed. The tuning-fork--or that Real which in relation to Sense is the particular object thus named--will, by one of its modes of acting on my Sensibility through my optical apparatus, determine the response known as _vibrations_; but it is not this response of the optical organ which is transformed into, or causes the response of the auditory organ, known as _sound_. The auditory organ knows nothing of vibrations, the optical nothing of sounds. The responses are both modes of Feeling determined by organic conditions, and represent the two different relations in which the Real is apprehended. The Real _is_ alternately the one and the other. And if the one mode of Feeling has a physical significance, while the other has a mental significance, so that we regard the vibrations as objective facts, belonging to the external world, and the sounds as subjective facts, exclusively belonging to the internal world, this is due to certain psychological influences presently to be expounded. Meanwhile let us fix clearly in our minds that both vibrations and sounds are modes of Feeling. My consciousness plainly assures me that it is I who see the one, and hear the other; not that there are two distinct subjects for the two distinct feelings. Add to which, manifold uncontradicted experiences assure me that the occasional cause--the objective factor--of the one feeling, is also the cause of the other, and not that the two feelings have two different occasional causes. From both of these undeniable facts we must conclude that the difference felt is simply a difference of aspect, determined by some difference in the modes of apprehension. 38. Assuming then that a mental process is only another aspect of a physical process--and this we shall find the more probable hypothesis--we have to explain by what influences these diametrically opposite aspects are determined. From all that has just been said we must seek these in the modes of apprehension. There can be no doubt that we express the fact in very different terms; the question is, What do these terms _signify_? Why do we express one aspect in terms of Matter and Motion, assigning the process to the objective world; and the other aspect in terms of Feeling, assigning the process to the subjective world?