skepticism

In contrast to realism, which posits that the human mind has an innate ability to see reality as it is, Locke, a skeptic, in the mid-1600s, posited that the mind starts out as a blank slate, and is developed through the experience written upon it in th form of sensory impressions that represent external material objects.

Hume, a radical skeptic, posited that the mind imposes order of its own on the sensations it receives from the real world in the mid-1700s. This order is simply an association of ideas, a habit of human imagination through which it assumes causal connections. There is nothing innate about knowing and the causal connections we postulate are simply accidents of repeated connections in the mind. Ideas result from connections in experience, not from an independent reality, and intelligibility reflects habits of mind, not the nature of reality. Hume claimed that there was no necessary order to our ideas other than the ways they were combined in our minds according to habit and the laws of association.

Knowledge and skepticism --
The implication of skepticism is that knowledge is relative and unreliable.

Scientific revolution implications --
The scientific revolution, starting somewhere around 1500 and continuing through 1800, depended on a key assumption -- that reasoning humans were capable of forming hypotheses about reality. Central to the scientific method is the individual scientist who objectively observes nature, formulates hypotheses about the laws governing it, and then tests those laws against quantified data, so progressively moves towards a fuller understanding of the laws. These laws are understood to take the form of universal, timeless, deterministic, linear 'if -- then' casual links. The notion of the scientific method spread beyond the realm of nature to such realms as social behavior and economics (Adam Smith). There truly was a scientific revolution taking place, putting, or appearing to put, knowing how everything works within the grasp of humans. Skepticism was raining on this parade, drawing into question the validity of the knowledge gained.

The other issue that arose from the scientific revolution, as it spread to all realms of things, is the question of human freedom and choice. Since humans are part of nature, they would also be subject to the deterministic laws of nature and not have freedom.

Kant's dualistic resolution --
Kant, around the mid-1700s, developed transcendental idealism as an alternative to realism, on the one hand, and skepticism, on the other.

Source: Stacey, 2007, pp 31-32