 On the question of universals, Buridan's main contribution is to develop what Gyula Klima has nicely termed an “essentialist nominalism”, arguing for the reality of the natures underlying our simple substance concepts. Ockham understood universality in terms of the inherent indifference of our cognitive acts, so that despite our having had different particular experiences of horses, say, we are able to form complex concepts that count as universal provided they are able to represent all horses; crucially, there is nothing of the substantial nature of a horse that is conveyed via the senses to the intellect on Ockham's theory. Against this, Buridan argues that substantial concepts cannot merely be collections of complex, connotative concepts, because then we would have the absurd conclusion that substantial terms are not substantial terms, for the concepts according to which they were imposed to signify would not be simple substance concepts: