All created beings are real, but they are real in degrees: they participate in being more or less fully. Pure matter without form is at the lowest extreme — potential being, not yet actualised. At the summit of creation stand the angels, purely intellectual beings whose form is not limited by matter and whose existence approximates most closely the divine simplicity. Between these extremes, the material world forms a continuous gradation from inanimate matter through plants to animals to human beings, each level possessing capacities — life, sensation, reason — that the level below lacks.
In Aquinas's hylomorphic metaphysics, form is the source of both the intelligibility and the being of a thing. Higher forms confer greater degrees of actuality on the matter they organise. The human soul, as a form that subsists in its own right — capable of intellectual operation independent of any bodily organ — occupies a unique position in the material hierarchy: the lowest of intellectual substances, but genuinely intellectual and thus oriented toward the entire order of being. The soul's natural desire to know all things reflects its position at the boundary between the material and the spiritual.
Aquinas insists that the hierarchy of being is good precisely as a hierarchy. A universe containing only one kind of being would be less perfect than one containing many kinds — the diversity of forms mirrors, through multiplicity, the inexhaustible perfection of a God who cannot be expressed by any single finite form. The apparent imperfections of lower grades of being are not defects in a poor design but constitutive features of a richer whole. What is limited in the part is ordered to what exceeds it; what is possible at each level is called upward toward the actuality that belongs fully only to the first.
The hierarchy of being pervades the Summa Theologiae and is condensed in On Being and Essence, Chapters IV–VI. Aquinas draws on Pseudo-Dionysius's Celestial Hierarchy and Aristotle's scala naturae, integrating them within a participatory metaphysics derived ultimately from Plato via Neoplatonism.
