Some men have minds particularly active in comparing and forming ideas. These are invariably the most ingenious, and, circumstances concurring, will always distinguish themselves. There are others, and in a greater number, whose minds are less active, allow all sensations which have not a certain degree of force to escape, and who only compare those by which they are strongly agitated. In points of ingenuity and vivacity these yield to the former. Others still there are, and they form the multitude, in whom there is so little activity of mind, so little propensity to think, that they compare and combine nothing, at least at the first glance; sensations of force, and repeated a thousand times, are required before their minds will be influenced to compare them, and form ideas.
The consciousness of our existence being composed, then, not only of our actual sensations, but of the train of ideas which gave rise to the comparison of our sensations, and of our past existences, it is evident that the more ideas we have, the more certain we are of our existence; that the more we have of intellectual capacity, the more we exist; that it is by the power of reflection alone that we are certain of our past existence, and view our future one; the idea of futurity being nothing more than a comparison of the present with the past inverted, since in this light the present is past, and the future present.
This power of reflection being denied to animals, it is certain they cannot form ideas, and consequently their consciousness of existence is less sure, and less extensive than ours. Having no idea of time, no knowledge of the past, nor conception of the future, their consciousness of existence is simple, depends solely on the sensations which actually affect them, and consists in the internal sentiment which these sensations produce.
May we not conceive what this consciousness of existence is in animals, by reflecting on our own state when strongly occupied with some object, or violently agitated by some passion, which banishes every reflection upon self? This state we familiarly express by saying, the man is absent or beside himself; and people are in reality beside themselves, when they are occupied with sensations actually present to them, especially if those sensations are so violent and rapid as to allow the mind no time for reflection. When thus situated we feel pleasure and pain in all their varieties; therefore, though seemingly without the participation of the mind, we have a consciousness of our existence. This state, to which we are occasionally exposed, is the habitual state of animals; deprived of ideas, and furnished with sensations, they _know_ not their existence but _feel_ it.