Substantialism

The decisive argument is that of the witness: whatever I can deny or objectify, the body, the sensations, the passing thoughts, I grasp as "that," distinct from the "I" that observes it; this witnessing subject can therefore never be reduced to the object it lights up, and the consciousness that would establish its own annihilation would have, in order to observe it, to remain. This Self-consciousness, the condition of all experience, is uncompounded, and what is uncompounded cannot fall apart: it is eternal, and its identity with brahman is the very teaching of the "great sayings" of the Upaniṣads. Here Advaita parts squarely from Buddhism: where anattā (non-self) sees behind the flux of aggregates only an impermanent series without a subject, Advaita retorts that no flux could appear as a flux without a stable consciousness running through it and attesting to it.

Bṛhadāraṇyaka Upaniṣad; Chāndogya Upaniṣad, VI, 8 (tat tvam asi) · Śaṅkara, Commentary on the Brahma-Sūtras (Brahma-Sūtra-Bhāṣya)

The Platonic self is a substance (a reality subsisting in itself, the permanent bearer of identity): the soul is a simple, immaterial and immortal reality, distinct from the body and akin to the Forms. It constitutes the person's true identity, the body being only a provisional garment from which it separates at death.

Plato, Phaedo, 78b-80b, the simplicity of the soul · Plato, Republic, 611b-612a

If the soul knows the eternal and unchanging intelligible, it is because it is akin to it: what grasps the simple must itself be simple. From this kinship Plato concludes that the true self is not the bodily composite but the soul, an immaterial substance that preexists birth and survives death. The Alcibiades sharpens the divide: a person is neither the body nor the compound of both, but "is" the soul, the body being merely an instrument it uses. This identification of the person with a separable substance, against any self bound up with embodiment, grounds the whole of Platonic psychology and eschatology.

Phaedo, 78b-80b · Alcibiades, 129e-130c

The self is not the bodily composite but the soul, an immaterial and immortal substance, and more deeply still the intellect within us. Plotinus presses inwardness to a singular thesis: the highest part of the soul never wholly descends into the body but abides permanently in the intelligible. Our true self is that unaltered summit, of which we have usually lost awareness, absorbed as we are in sensible life. To know oneself is to awaken to this higher identity, more real than the empirical person.

Enneads, IV, 8, 8 (the undescended soul) · Enneads, I, 1 (What Is the Living Being, and What Is Man)

The true self, the ātman, is neither the body, nor the mind, nor the changing sequence of psychic states: it is pure consciousness, the permanent ever-present witness that every experience presupposes. It cannot be denied without being confirmed, for the one who would deny it would still be it. Śaṅkara thus takes the exact opposite of the bundle thesis: where one would find only a flux of impressions without a bearer, as Hume would hold, or before him the Buddhist not-self (anātman), he maintains an irreducible subject. Yet this permanent core is not one individual soul among others: it is identical in all and identical to brahman, which radicalizes substantialism while depersonalizing it.

Upadeśasāhasrī, II (prose part), on "thou art that" · Commentary on the Bṛhadāraṇyaka Upaniṣad

The self is a substance, a thinking thing, whose existence is grasped immediately in the cogito and whose identity depends on neither body nor memory. This is a substantialism of the subject, which also secures the soul's immortality.

Meditations on First Philosophy, II · Discourse on Method, IV

If bodies are passive, made of inert ideas, then that which perceives and wills must be of a wholly different nature: an active, simple and indivisible substance, which Berkeley calls spirit or soul. The self is therefore not one idea among ideas but that which has them and acts upon them, known not by perception but by a notion we have of our own activity. Here is a sharp break with Hume, who in introspection will find only a bundle of perceptions with no subject: for Berkeley, to remove the perceiving subject would be to remove that without which no idea would exist at all. The self as substance is saved at the very point where matter as substance is denied.

Principles of Human Knowledge, §§2, 27 and 139-142 · Three Dialogues between Hylas and Philonous, III

The self is a monad, a simple, indivisible substance whose individual notion includes once and for all everything that will ever happen to it. Personal identity is thereby secured by the permanence of this substance, a marked substantialism. Leibniz nonetheless grants a role to memory and the continuity of consciousness (apperception), adding a nuance of psychological continuity.

Monadology, §1-3 and §14 (the monad, simple substance; perception and apperception) · Discourse on Metaphysics, §8-9 (the individual notion of each substance)

Against the Platonic dualism that makes the soul a pilot lodged in the body, Thomas holds the soul to be the form of the body in the Aristotelian sense: the human being is not a soul using a body but a substantial unity, a single being whose soul is the formal principle. The self is therefore a substance, but a hylomorphic substance, not a pure spirit. The difficulty proper to Christianity, personal immortality, is resolved by making the human soul a form indeed united to the body yet subsistent, capable of operations (intellection) that do not depend on an organ, and thus able to survive death while awaiting the resurrection.

Summa Theologiae, Ia, q. 75-76