  It does not follow from these definitions that anything which inheres in the substrate is a non-substrate cause, or that anything which does not is an instrumental cause. In other words, we still need to define “being a cause”. First, however, two more types of causal factor are distinguished by the Nyāya. These are the ‘special’ instrumental cause (karaṇa) and the ‘operating condition’ (vyāpāra) We often speak of the cause of an effect, and in doing so pick out one factor from the total collection of causal factors as being particularly salient or important in bringing about the effect. Intuitively, the idea is that, even if we have the substrate cause (the pot-halves), the non-substrate cause (the contact between the halves), and even some of the instrumental causes (the potter, etc.), we will not get the effect (the pot) unless the potter acts or ‘operates’ to bring it about. In a sense, then, it is the potter’s actually using his stick which is the cause of the pot. The Naiyāyikas try to capture this idea by saying that one of the instrumental causes is the ‘special’ (asādhāraṇa) cause of the effect, and they call it the ‘karaṇa’. This notion is particularly important to the Nyāya epistemology (see below). What constitutes the ‘special’ nature of this cause? The Navya-nyāya (cf. TS 54) answer is as follows. They introduce first the notion of an ‘operative condition’ (vyāpāra), defined as that cause which immediately precedes the effect. For example, the operative condition for a pot is the conjunction of the pot-halves; for the felling of a tree, it is the final contact between axe and tree before the tree falls. The ‘special’ cause is defined as that factor which ‘brings about’ this operative condition. In other words, it is the factor whose operation directly brings about the effect. In the examples, it would be the axe or the potter’s stick. These objects are, for the Nyāya, the true instruments by means of which the effect is produced.