Immanuel Kant wrote philosophical treatises during the Enlightenment that dealt with logic, empirical evidence, sensory input, and epistemology, or how we learn what we know. We can situate his thoughts in an era interested in the scientific method, religion and metaphysics, ethical societies, and rivaling belief systems. Kant responded to Empiricists and Rationalists by asserting his thesis that everything we believe must be filtered through our senses and mind. Thus, truth is determined by how we understand and orient knowledge of an object, regardless of what other "independent" characteristics the object might possess.
During his long life, from 1724-1804, Immanuel Kant made numerous contributions to modern philosophy, developing what he called "transcendental idealism." He spent his entire life in his birthplace of East Prussia, working at the local university, reading contemporary Enlightenment philosophers, and writing treatises in German. He commented on everything from the existence of God to the aesthetics of beauty, as well as examining how entrenched, official bodies of knowledge influence the average person. Always wanting to promote freedom, independence, and equality, his theories encouraged individuals to take responsibility for determining their own truths.
Kant synthesized the philosophies of two opposing schools of thought to lay the groundwork for transcendental idealism. In many ways, his theories anticipated how 20th century physics understands observation, as well as how neurologists conceive of consciousness today. The Empiricists, especially David Hume, believed that we only can rely on what we know about the world through direct, experiential information, including emotions and frames of reference. However, the Rationalists encouraged logical knowledge that was accumulated, deduced, or concluded from other abstract knowledge, because this was independent from person to person variation.
A direct response to David Hume, Kant wrote The Critique of Pure Reason in 1781. In this seminal work, he argued that we are, by necessity, constrained in our perception of the world. These constraints, or filters, are primarily our senses and the organization of our thoughts as determined by our brain's physiology. We acquire knowledge both organically and abstractly, but the process of acquisition cannot be separated from the knowledge itself.
The constraints of which Kant spoke do not have a negative connotation. They can be thought of as filters, models, language, instruments, consciousness, or the limits of our imagination that structure the tenacity of beliefs. It follows that if there are metaphysical, or supersensory, phenomena, humans do not have access to them. Without arguing that the world is an illusion, Kant still reasons that all its objects are fundamentally unknowable as independent from our involvement in their observation.