10. When we perceive an impression we never call it a substance, if we refer it to an object, or consider it as objective: for the object is not, of itself alone, capable of connecting various sensations. We receive the sensations of red, and not only ordinary people, but even philosophers, when not philosophizing, make the color objective, and consider the red, not as a simple sensation, but as an external quality. No one would call this quality by itself a substance; for it is not capable, of itself alone, of connecting other impressions or qualities. If there is a change of color the red disappears, and the new impression is connected in the order of time with the sensation of the red, but does not reside in it. If there is a change of form, although the red continues, we do not conceive this color as the necessary link between the two forms, because we know that the continuance of the red is indifferent to the variety of form, which may be changed with or without the continuance of this color. As in general we have experienced that no sensation is necessarily connected with another, and that among sensations connected at a common point, some disappear without the rest disappearing, we infer that none of them is a necessary link; and therefore, although we make them objective, we do not give them the character of a substance, of any thing remaining identical through changes, of which it is, as it were, the recipient. 11. There is a property in bodies which is necessary to all sensations, or at least, to the two principal sensations of sight and touch. This property is extension, which, whether considered subjectively or objectively, we regard as a recipient of all sensations. We neither see nor imagine the white or black; we neither touch nor imagine the hard or the soft, the warm or the cold, without the extension in which the whiteness or blackness, the hardness or softness, the warmth or the cold reside. Thus extension might perhaps merit the honor of substance, if it were not subject to another condition, which deprives it of this title.