Excerpt:
"Kant...accepted the notion of things-in-themselves existing independently of any knowledge.... As his starting point [he took it] that any specific knowledge we claim to have of such and such an external object is obtained through our senses, [and] hence is at best only indirect and questionable.... What we know directly and with certainty is therefore only the set of our ideas. [For example,] the very notion of causality [is] an a priori mode of human understanding," in other words, an idea (d'Espagnat 1995:5-7).