Leibniz speaks no less distinctly and sanely in this direction. "True powers are never mere possibilities: they are always tendency and action." The "Monad"--that is the quasi-intelligent unit of existence,--is essentially activity, and its actions are perceptions and appetitions, i.e. tendencies to pass from one perceptive state or act to another. It is out of the variety, the complication, and relations of these miniature or little perceptions and appetitions, that the conspicuous phenomena of consciousness are to be explained, and not by supposing them due to one or other faculty. The soul is a unity, a self-developing unity, a unity which at each stage of its existence shows itself in a perception or idea,--each such perception however being, to repeat the oft quoted phrase, _plein de l'avenir et chargé du passé_:--each, in other words, is not stationary, but active and urgent, a progressive force, as well as a representative element. Above all, Leibniz has the view that the soul gives rise to all its ideas from itself: that its life is its own production, not a mere inheritance of ideas which it has from birth and nature, nor a mere importation into an empty room from without, but a necessary result of its own constitution acting in necessary (predetermined) reciprocity and harmony with the rest of the universe.
But Hobbes, Spinoza, and Leibniz, were most attentively heard in the passages where they favoured or combatted the dominant social and theological prepossessions. Their glimpses of truer insight and even their palpable contributions in the line of a true psychology were ignored or forgotten. More attention, perhaps, was attracted by an attempt of a very different style. This was the system of Condillac, who, as Hegel says (p. 61), made an unmistakable attempt to show the necessary interconnexion of the several modes of mental activity. In his _Traité des Sensations_ (1754), following on his _Essai sur l'origine des connaissances humaines_ (1746), he tried to carry out systematically the deduction or derivation of all our ideas from sense, or to trace the filiation of all our faculties from sensation. Given a mind with no other power than sensibility, the problem is to show how it acquires all its other faculties. Let us then suppose a sentient animal to which is offered a single sensation, or one sensation standing out above the others. In such circumstances the sensation "becomes" (_devient_) attention: or a sensation "is" (_est_) attention, either because it is alone, or because it is more lively than all the rest. Again: before such a being, let us set two sensations: to perceive or feel (_apercevoir ou sentir_) the two sensations is the same thing (_c'est la même chose_). If one of the sensations is not present, but a sensation made already, then to perceive it is memory. Memory, then, is only "transformed sensation" (_sensation transformée_). Further, suppose we attend to both ideas, this is "the same thing" as to compare them. And to compare them we must see difference or resemblance. This is judgment. "Thus sensation becomes successively attention, comparison, judgment." And--by further steps of the equating process--it appears that sensation again "becomes" an act of reflection. And the same may be said of imagination and reasoning: all are transformed sensations.