Aquinas on Necessity

‘It should be noted that there are two kinds of necessity, namely, absolute necessity and conditional necessity. That necessity is absolute which proceeds from prior causes in the order of generation, and these are the material and the efficient causes; for example the necessity of death which comes about from matter, namely from the disposition of contrary components–and it is called absolute because there is no impediment to it. This necessity is also called the necessity of matter. On the other hand, conditional necessity proceeds from causes which are posterior in generation, namely, from the form and the end; for example, we say that it is necessary that there be conception if a man is to be generated. And this necessity is also called conditional, because it is not absolutely necessary that this woman conceive but only under this condition, namely, if a man is to be generated. And this necessity is called the necessity of the end.’ (Thomas Aquinas, ‘On the Principles of Nature)

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