Thus it appears that the five senses are the organs of sensation, and that through their instrumentality material things operate upon the mind. _Perception_ is a _sensation_ attended by the _belief of a cause_, and it is this additional circumstance alone which distinguishes perception from sensation. If a person were asleep, and should suffer from the prick of a pin, or be disturbed by an unpleasant sound, these would be mere sensations, because the mind would not ascribe them to any cause. But if the person should waken, these sensations would immediately become perceptions, because they would be attended by the belief of some cause. _Conception_ is a state of mind similar to perception, and differs from it in being less vivid, and in not being produced through the medium of the senses. When we look at a tree, we have a _perception_ of this object. But the mind can also have an idea of this tree when removed from the sight, though the idea is not so vivid and distinct, nor have the senses any agency in producing it. The perfume of a rose, also, occasions another sensation; but when the rose is removed, so as not to affect the senses, we can still have a _conception_ of its perfume. The conception differs from the perception only in being less vivid, and in not being caused by a material object acting on the senses. _Memory_ is either a conception or a perception, which is attended with a feeling of its resemblance to a past state of mind. It is this feeling of resemblance that is the only circumstance which distinguishes memory from conception. Thus we may conceive of a tree without recognizing it as the particular idea of any tree we may have seen before; but if this is accompanied by a feeling of the resemblance of this idea to the one we always have when we see the tree that shadows the paternal roof, this conception becomes _memory_. If we conceive the form of a man without recognizing the resemblance of this idea to the perceptions we have when we see any particular man, this is a simple act of conception; but if we recognize in this object of conception the features of a dear friend, this act then becomes memory. Again, if we conceive of certain events and circumstances attending them without recognizing this combination as ever having existed in past experience, they are mere conceptions; but if we recognize in them the events and circumstances of past experience, conception becomes memory. _Imagination_ is the power which the mind possesses of arranging our conceptions in new combinations. We can conceive objects as united together of which we never conceived before as thus united.