It is sui generis, unique, belonging to its own category, distinct from all others. If reason is any kind of event at all, it is a human event. The ability to do sustained reasoning is a distinctly human ability. As for its place in causation, sustained reasoning, or the results of it, joins the chain of causation as a cause, not an effect. When thought is performing its proper office it is determinative, not determined.
Some qualifications are in order, of course.
Undoubtedly, all sorts of things may influence, disrupt, distort, or even condition our thinking. Intense heat or cold may make any but the most elementary thinking extremely difficult. Low pressure systems in the atmosphere may depress us, and high pressure systems may exhilarate us. Our wishes, whether born of material interests or immaterial desires for diversion, may lead us to wrong conclusions. Pathological mental states may render us incapable of sound reasoning.