Kant builds on the work of empiricist philosophers such as John Locke and David Hume, as well as rationalist philosophers such as Gottfried Wilhelm Leibniz and Christian Wolff. He expounds new ideas on the nature of space and time, and tries to provide solutions to the skepticism of Hume regarding knowledge of the relation of cause and effect and that of René Descartes regarding knowledge of the external world. This is argued through the transcendental idealism of objects (as appearance) and their form of appearance. Kant regards the former "as mere representations and not as things in themselves", and the latter as "only sensible forms of our intuition, but not determinations given for themselves or conditions of objects as things in themselves". This grants the possibility of a priori knowledge, since objects as appearance "must conform to our cognition...which is to establish something about objects before they are given to us." Knowledge independent of experience Kant calls ""a priori"" knowledge, while knowledge obtained through experience is termed ""a posteriori."" According to Kant, a proposition is "a priori" if it is necessary and universal. A proposition is necessary if it could not possibly be false, and so cannot be denied without contradiction. A proposition is universal if it is true in all cases, and so does not admit of any exceptions. Knowledge gained "a posteriori" through the senses, Kant argues, never imparts absolute necessity and universality, because it is always possible that we might encounter an exception.