_Other Perceptions attributable to the same Source._--So with regard to externality, extension, and form. When an external object, a cube, for example, or an ivory ball, is placed on the palm of the hand, sensation is awakened, but is that sensation necessarily accompanied with the perception of the external object as such? Does the mere tactual sensation, in the first instance, and of itself, inform us that there is something external to ourselves, that what we feel is not a part of our own organism? We are conscious of a change in the sensation of the part affected, but are we immediately conscious that this change is produced by something external? Let there be given, however, the consciousness of resistance to our muscular movements, as when the cube or ball, for instance, prevents the effort to close the hand, or when our locomotion is impeded by the presence of some obstacle, and will not the same resistance inform us of the extension of the resisting body, and so of its form and figure? We learn whereabout in space this resistance occurs, and where it ceases. The tactual sensation would indeed very soon come to our aid in this cognition, and serve as a guiding sense, even in the absence of the former. The question is, whether this alone would, in the first instance, give us such cognitions? _Our first Ideas of Extension, how derived._--We have had reference in this discussion only to the qualities of _external_ bodies. There can be little question that our _first_ ideas of extension are derived from our own sentient organism, the consciousness of sensations in different parts of the body, distinct from, and out of each other, thus affording the knowledge of an extended sentient organization. The idea of externality, or outness, and extension, thus acquired, the transition is easy from the perception of our own bodies as possessing these qualities, to the cognizance of the same qualities in external objects. § VI.--CREDIBILITY OF OUR SENSATIONS AND PERCEPTIONS. _Denied by some._--There have always been those who were disposed to call in question the testimony of the senses. Such were the Eleatics and the Skeptics among the Greek philosophers, and there have not been wanting among the moderns minds of acuteness and ingenuity that have followed in the same path. While admitting the _phenomena_ of sense, the _appearance_ of things as being so and so, they have called in question the corresponding objective reality. Things appear to me to be thus and thus--such and such impressions are made on my senses--that I cannot deny; but how do I know that the reality corresponds to my impressions, or, in fact, that there is any reality? How know we our senses to be reliable? What evidence have we that they do not habitually deceive us?