 Ockham picks up on this idea. He does not deny that we can know of some causes by simple reasoning, on the basis of self-evident or well-established metaphysical principles. We can know, for instance, that the will causes its own acts (Ockham Ord. I dist. 1, qu. 3) or that there exists a first conserving cause of this world (Ockham Ord. I dist. 2, qu. 10]). But when it comes to the discovery of causes within the natural world, we must rely on observation. If a causal agent is of a very specific kind (in Ockham’s terms, known by way of its species specialissima), just one observation will suffice. A single observation of a particular kind of herb curing a particular illness, for example, can safely be generalized. It can be generalized on the basis of the principle that “all agents of the most specific kind will bring about effects of the same nature” (Ockham Ord. I prol. qu. 2). If the suspected cause is of a more generic kind, then many instances will be needed (Ockham Ord. I prol. qu. 2). But in either case, the process is thoroughly empirical: it involves the isolation of causal factors by way of experience. As Ockham writes,