Probability is the cornerstone of modern scientific method, but is only referred to by Locke as a question of plausibility. Hume relates the strength of our belief that a particular cause will lead to a particular effect to the frequency with which we have seen it to be so (ibid., p. 47), even though he argues quite specifically that this tendency to generalise is a part of human nature and not rational (ibid., p. 38) and admits himself at a loss to explain it or to find a concurrence with other philosophical concepts. The nub of his scepticism on this point is that we cannot know for certain what will occur simply on the basis that it has not occurred before. This is later referred to by Karl Popper (1979) as “Hume’s problem”; Popper’s solution to Hume’s problem (the concept of falsifiability) had far-reaching effects on scientific methods.

Finally, Hume follows the logic of empiricism to a profoundly different conclusion, in relation to knowledge of ourselves, from the Cartesian dualist conception. In his Treatise ef Human Nature (Cahn, 2002, p. 803) Hume notes that “When my perceptions areremoved for any time, as by sound sleep; so long am I insensible of ‘myself’ and may truly be said not to exist”. From this observation he goes on to argue that those who actually experience a ‘self’ are suffering an illusion. In this sense, his scepticism extends further than Descartes’, doubting even the continuity of  the self. As a  materialist,  Hume proposed that there is no ‘self’  that is independent of our perception. Instead, the impression of self is given through successions of different perceptions that are in perpetual flux. To modern sensitivities, in part because of the subsequent influence of Kant, it is difficult to accept this extreme conceptualisation of the self.

Kant (1724 -1804)

Kant’s work can be seen as  a  revolutionary  bringing together of rationalism and empiricism. Whereas for Descartes all our knowledge was given by God and available to us only through our ratiocinations, and for Hume all we could know was through our senses, for Kant the truth was not ‘out there’ in some form waiting to be discovered but instead proposed the only knowledge that we could have is created by us via the functions of our minds. For Kant ‘reality’ is organised and made intelligible through forms of perception – understanding and imagination that are hard wired into the experiencing mind. So, whilst Descartes had over-emphasised the role of reason in knowledge creation and Hume over-emphasised sense-perception, Kant argued that there should be no dichotomy: perception without conception (i. e., reason) was blind, and conception without perception was empty.

Thus, Kant’s composite theory introduces knowledge as a more complex process than hitherto presented. The sensory element is important, but mind is more than a passive recipient. Kant further submits that our minds actually impose forms or categories, such as space and time, upon our sensations, thus rendering all possible experience coherent. He calls the categories ‘pure concepts of the understanding’ (Cahn, 2002, pp. 960-965) and suggests they are essential for knowledge generation. The notion of these concepts of understanding has a significant impact on all philosophical thought that follows. For Kant, then, the mind is a complex set of a priori concepts and is not the blank sheet of the empiricists.

Whereas Hume had argued that the basis of our actions was founded in habit and that we build up knowledge over a period of time from our experience, Kant was not satisfied with this explanation and almost echoing Locke’s concern, questioned how and why we experience things in the first place. He claimed that the conditions for our understanding were in fact synthetic (i.e., produced when the mind determines the conditions of its own experience) and a priori, enabling us to categorise and order the world. It follows from this that our descriptions of the world necessarily have to conform to the perceptual tools of the mind provided to us. Kant gives space and time and cause and effect, as examples of concepts (categories) that are synthetic and a priori and   not empirically derived. We use these to  categorise, order and shape the world around us. Accordingly, Kant’s ‘categorical imperative’ gives an answer, of sorts, to Hume’s question of why we seek and find relationships between events.