 Another influential idea was the distinction between logical and natural necessities and possibilities. In Scotus’s theory, logically necessary attributes and relations are attached to things in all those sets of compossibilities in which they occur. Against this background one could ask which of the natural invariances treated as necessities in earlier natural philosophy were necessary in this strong sense of necessity, and which of them were merely empirical generalizations without being logically necessary. (For a discussion of logical and natural necessities in the fourteenth century, see Knuuttila 1993, 155–160, 2001a.) Buridan distinguished between logical and natural necessities in his classification of four grades of necessity (Dialectica 8.6.3). He used a non-extensional metaphysical notion of possibility sencundum imaginationen in his questions on Aristotle’s Physics (Sylla 2001), but a frequency interpretation in his questions on Aristotle’s On generation and corruption I.4 (57). This was the main modal conception in the Averroist Aristotle commentaries by John of Jandun (see, for example, his questions on Metaphysics, IX.5 114va-b; questions on On the Heavens I.34, 21vb) and remained a popular option until the Renaissance times (Knebel 2003). 