Arthur Schopenhauer, answered from the texts and cited to the page.
The principium individuationis — the principle of individuation — is the name I borrow from the Scholastics to designate the role that space and time play in generating the appearance of multiplicity from what is, at its inner core, a single essence.1 Here is what I mean precisely. The will, as thing in itself, lies entirely outside the forms of appearance — outside space, outside time, outside causality, outside the whole architecture collected under the principle of sufficient reason.2
It is one. But the moment it appears, it appears through space and time, and space and time are what make it possible for something that is one and the same in essence to present itself as many — as a multitude of coexistent and successive individuals.3 Space and time are thus the principium individuationis: the mechanism by which the unitary will fractures, in appearance, into the countless separate beings that populate the phenomenal world.
The consequence for ordinary human experience is this: the crude individual, as the Indians say, has his eyes clouded by the veil of māyā; he sees not the thing in itself but only appearances in time and space, in the principium individuationis.4 He takes the boundaries between himself and others to be metaphysically real, and from this conviction egoism grows naturally — the sense that "I have my true being in my own self alone, and everything else is not-I and foreign to me." But individuation is mere appearance, arising by way of space and time, which are nothing more than forms of our faculty of intuition and belong to it rather than to things as they are in themselves.5
Kant demonstrated this in the Transcendental Aesthetic with a clarity that has never been successfully contested, and it is among the extremely few genuine conquests in the field of metaphysics: if space and time are foreign to the thing in itself, then plurality is foreign to it also.67 In all the countless individuals of this world, only one and the same essence truly manifests itself.
This is not merely a metaphysical curiosity. The recognition that individuation is appearance — that my true inner essence exists in every living thing as immediately as it reveals itself to me in my own self-consciousness — is precisely what erupts as Mitleid (compassion, literally "suffering-with"), and upon compassion rests all genuine, disinterested virtue.8
The formula the Sanskrit tradition gives for this recognition is tat tvam asi ("you are that"): when you see another suffering, what you are seeing, if you see truly, is yourself.9 Individuality, then, inheres only in the affirmation of the will, not in its negation. It belongs to the intellect insofar as the intellect reflects appearance and assumes the form of the principium individuationis; but in the negation of the will — in what the Buddhist tradition calls nirvana — character itself is abolished, and the numerical identity of the inner essence of all living things becomes not merely a philosophical thesis but the actual condition of the one who has ceased to will.10
I call space and time, in respect of their making plurality possible, the principium individuationis, unconcerned whether this is precisely the sense in which the Scholastics took this expression.The Two Fundamental Problems of Ethics, pp. 250–251
As thing in itself, the will is completely different from its appearance, and entirely free of all forms of appearance. The will only takes on these forms when it appears, which is why these forms concern only its objecthood and are foreign to the will itself.The World as Will and Representation Vol 1, p. 137
It is only by virtue of time and space that something that is one and the same in essence and concept can nonetheless appear as different, as a multiplicity of coexistent and successive things: time and space are thus the principium individuationis.The World as Will and Representation Vol 1, p. 137
the eyes of the crude individual are clouded, as the Indians say, by the veil of māyā: it is not the thing in itself that shows itself to the individual, but only appearances in time and space, in the principium individuationis.The World as Will and Representation Vol 1, p. 378
'Every individual is a being fundamentally distinct from all others. I have my true being in my own self alone, and everything else, on the contrary, is not-I and foreign to me.' - This is the knowledge to whose truth flesh and bone bear witness, that lies at the ground of egoism.The Two Fundamental Problems of Ethics, pp. 253–254
'Individuation is mere appearance, arising by way of space and time, which are nothing more than the forms of all objects of my cerebral cognitive faculty and are conditioned by it; so the plurality and distinctness of individuals is also mere appearance, i.e. is present only in my representation.'The Two Fundamental Problems of Ethics, pp. 253–254
if time and space is foreign to the thing in itself, i.e. to the true essence of the world, then necessarily plurality is foreign to it also: consequently in the countless appearances of this world of the senses it can really be only one, and only the one and identical essence can manifest itself in all of these.The Two Fundamental Problems of Ethics, pp. 250–251
'My true, inner essence exists in every living thing as immediately as it reveals itself in my self-consciousness to myself alone.' - It is this knowledge... that erupts as compassion, upon which, therefore, rests all genuine, i.e. all disinterested virtue.The Two Fundamental Problems of Ethics, pp. 253–254
It is this knowledge, for which the standing expression in Sanskrit is the formula tat-twam asi, i.e. 'You are that'.The Two Fundamental Problems of Ethics, pp. 253–254
Individuality inheres firstly in the intellect, which reflects appearance and in virtue of this belongs to appearance, which itself assumes the form of the principle of individuation... Individuality therefore inheres only in the affirmation of the will, not in its negation.The World as Will and Representation Vol 2, p. 675