By the time of the mid-eighteenth century a diversity of metaphysical theories had been proposed. In summary, Locke emphasized the role of the external object, attributing to the actual extant object both primary qualities, those qualities that are actually part of the existing object, and secondary qualities, those qualities that exist in the perception of the observer. Leibniz proposed the theory of "individual substances", designating them as "monads" and describing them as "active centers of energy." With Berkeley's proposal emerged the "total denial of material substance" while placing an emphasis upon the role of the perceiver and, in so doing, invariably linking the existence of an object with the perception of that object. (John Cottingham, Western Philosophy: An Anthology, p. 102)
David Hume introduced a new level of ontological and epistemological skepticism. Hume asserted that the notion which Locke had referred to, of an unknown "something" supposed to "support qualities” was an "unintelligible chimera". Hume proposed that our knowledge of an object cannot go beyond our experience and further that experience is no guarantee of what may be known or how we may know reality. He argued that our ideas of bodies are nothing but collections of ideas formed by the mind.
Hume divides all the objects of legitimate human inquiry into two classes which he called 'Relations of Ideas" and "Matters of Fact". Relations of Ideas, typified by the truths of mathematics, are established a priori or independently of experience and they form a closed system, arising merely from how our ideas or concepts are defined. Relations of Ideas do not provide us with any information about what really exists in the world.
Matters of Fact, by contrast, are established a posteriori and are concerned with what really exists. However, neither propositions of this kind can be conclusively demonstrated and to posit the falsity of such propositions is not contradictory. For example, that the sun will not rise tomorrow is no less intelligible and implies no more contradiction than that it will rise tomorrow according to Hume. Instead such truths are based entirely on experience and all that experience reveals is what has actually been observed to happen up until now. While an event may have transpired in the past there is no guarantee the event will necessarily occur again in the future.
Hume therefore emphasizes our inability to predict, in advance of experience, how even the most familiar objects will behave. For Hume, we can never predict that "A" will cause "B". We can never say that "A" produced "B". We can never establish the relationship of cause and effect. The most we can say is that two events, which are perceived as "cause and effect" occurred in close proximity of time.
Consequently, we can never predict what will occur in the future.
For example, Hume proposed that, for all we know, when one billiard ball hits another, both balls might remain absolutely at rest. This has implications for science and the scientific method. Hume argued that while the scientist may endeavor to reduce all events to a set of laws which will establish what will occur in the future when steps A and B are taken, this is in actuality a futile endeavor since we can never know that such what will be repeated in the future even if past experience, or the scientific method, has repeatedly demonstrated the legitimacy of our conclusions regarding what may occur in the future.
The closed a priori reasoning of mathematics, on the one hand, and the limited results of actual observation on the other, exhausts the proper sphere of human inquiry. Any metaphysical speculation which tries to go beyond these boundaries should be committed "to the flames" according to Hume, because it can contain nothing but sophistry or illusion.
