國立臺灣大學哲學論評

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During the Middle Ages, the two problems which philosophers concern
the most are the holistic unity of man and the immortality of soul. In order to
explain these basic Christian beliefs, philosophers adopt the dualism and
universal hylomorphism. The reason to take such a stance is to ensure the
immortality of soul.
For Aquinas, the human rational soul is both the substantial form of
body and the substance in its own right, and the holistic unity of man
requires the unity of the substantial form. Since the rational soul is created by
God, its esse (being) participates in Ipsum Esse. Rational soul is the essence
of humans, but not its complete species. It needs to be united with a body. It
is spiritual by nature. Once the rational soul is created, it has being, it is a
subsistent substance, but not exists separately by itself. The being of the
rational soul then unites the composit man’s soul and body. Being is the
unifying principle. It has absolute priority. There are double composition of
potency and act in man’s ontological structure. On metaphysical level, the
composition of being-act (or actus essendi) and essence-potency. On physical
level, the soul (act-form) and body (potency-matter), yet the rational soul is
essentially independent from body. The being of the rational soul is its
metaphysical foundation of immortality.