Immanuel Kant was a German philosopher from the 1700s. His contributions to metaphysics (questions relating to the nature of reality) and epistemology (questions relating to knowledge) profoundly shaped all philosophers who came after him. This is all because he went about philosophy in a unique way.
(A quick precursor: it’s worth mentioning that until the 1700s, philosophy was basically the same thing as science. Until scientific tools like microscopes and telescopes came about, the best way to understand more about our empirical world was to just think about it. So, for most of human history, philosophers were the same as scientists. It’s worth keeping that in mind as you read on!)
Before Kant, philosophers had tried to answer questions such as: what is the soul? Is all matter made of the same substance? What can we have knowledge of? And does God exist?
Kant, instead, began by asking a more fundamental question: how do we attain knowledge of the world? What conditions must be satisfied in order to have any knowledge at all?
Kant claimed that we could only enquire about the nature of our world when this was answered satisfactorily. Kant’s answer to this question, and the consequences of his answer, form what’s known as his ‘critical philosophy.’