The analysis of the text of St. Thomas Aquinas shows, that he was influenced by the Aristotelian notion of succession of the souls. It means that the human embryo is infused at the moment of conception, first with vegetative soul, then with the sensitive souls, and finally with a rational soul. When Aquinas' metaphysical views are considered in accordance with the contemporary biology, the theory of simultaneous animation, i.e. the infusion of the soul by God at the moment of conception, seems to be the best justified theory. Aquinas' description of a woman as mas occasionatus was also influenced by Aristotle's biology: the women are generated as men not in respect of particular nature, but only in respect of the universal nature. That is why, in comparison with men, Aquinas attributes a lower dignity to the women. Such an evaluation of women was not based on the metaphysical principles used for the explanation of the human beings, but it was the result of the influence of Aristotelian biology. When one uses the contemporary biological data in accordance with Aquinas' fundamental metaphysical principles, gender equality in human nature between men and women is convincingly justifiable.