* Philosophical realism: Aquinas believed that humans were composed of two parts: "prime matter" and "substantial form." The prime matter of humans is our body, and the substantial form is our soul. The soul is therefore not made of matter. Our souls are unique; there can be no two angels or humans that bear the same substantial form, or soul.
* Moral objectivism: The nature of the universe and essences of objects do not depend on the free will of God, but on His intellect, and ultimately on His essence, which is unchanging. The natural law, springing from the mind of God, is therefore immutable. Consequently, immoral acts are immoral not simply because God forbids them, but because they are inherently immoral. (Zigliara, "Sum. phil." (3 vols., Paris, 1889), ccx, xi, II, M. 23, 24, 25)
* Teleology: the universe is guided by principles, purpose, and design beyond the universe itself; specifically, the principles and design of God.
* Free will: Decisions are made by the interaction of the will and the intellect; the will presents objects to the intellect, and the intellect directs the will. Acts begin with the apprehension of the good in general by the intellect. We desire happiness naturally and necessarily, and not by free will; however, we choose particular goods freely. The will is a blind faculty, always following the past determination of the intellect. (Zigliara, 51).
* Senses: The senses are passive, in that they perceive, rather than creating, their objects. However, the will controls the exercise of the faculties, and thus determines and shapes what they perceive and how they perceive it.
* Empiricism: Held to the Peripatetic axiom: "Nothing is in the intellect that was not first in the senses," modified by saying that the intellect can ascend to the knowledge of higher things from the basis of perception, even God, and that the soul knows of its existence by its action.
* First principles: the basis for human knowledge is latent in the soul, not in the form of objective knowledge, but in the form of subjective inclination to believe them due to the evidentiary support: As soon as they are proposed they are known to be true. (Zigliara, op. cit., pp. 32-42).
* Universals: Universals are the primary object of the intellect, and are formed by abstraction from sense perception. The process of abstraction is so elevated above material conditions as to prove that man is spiritual.
* Immortality: The human soul is immortal by its very nature, because it has no principle of disintegration (Zigliara, p. 9).
* Arguments for the existence of God are made a posteriori, rather than a priori. In other words, the existence of God can be proven through perceptions and reason, but cannot be known by any innate knowledge. Thus the ontological argument is rejected, but several other arguments are made for the existence of God; these are considered in the next section.
