This distinction of actuality and potentiality is the saving of philosophy. Even physical science in our day has found 'potential' a convenient term. The distinction is heedlessly abolished by those who put activity for being, and seem to think that the human mind itself would perish the moment it ceased to act, as though there could be no reality that was not actualised. But perfect actuality can be nothing less than God: so that if actuality alone exists without potentiality, God alone exists. Nature by the institution of sleep teaches us to distinguish the potential from the actual. If mind may be dormant and yet not cease to be, so may the objects of mind be dormant -- unobserved by human sense, unpictured in human imagination, unrecalled in human memory, or even wholly out of the ken of human knowledge, -- and still really and truly be, as "permanent possibilities of sensation" or of cognition. This phrase of J.S. Mill is felicitous, if we remember, as he did not, that a "permanent possibility" is something raised above nothingness. Here then we have the confutation of idealism, of Berkeley and Kant and all their tribe. Phenomena, or appearances, cannot be actual to man except as objects of sensation or other human cognition: but they may very well be and are potential, observable though unobserved, out of all human mind. Potentiality however cannot be mere potentiality: it must rest on something actual. The actuality on which potential phenomena, appearances or accidents rest, is the substance in which they inhere.
The horns then of idealism are broken. Subject is not percipere; object is not percipi. If any one claims the liberty of using such a terminology, he must at least be brought to an admission that there is much of Mind which is not subject in his sense, and much of Matter that is not object. Mind and Matter are like sea and land, two vast potentialities. They meet on the coast-line: but the coast-line of percipere and percipi is far from being the whole reality.
Of God and His Creatures: 1.22