When reason and scientific thought were dominant, when Hume's skeptical ideas were widely accepted and religion and idealism were steadily losing ground, that's when Kant appears on the scene and gives a long-awaited, enlivening shot into the arm of metaphysics. Kant himself claimed that it was thanks to Hume that he was awoken from his “dogmatic slumbers.”
Up to that point, Kant was a follower of Leibniz, but with Hume human knowledge and experience came under threat, and life offered no certainties or truths. As a result, Kant made it his philosophical mission to reconcile scientific thought with the not trendy unscientific Platonic notions of the times and to rescue human knowledge from the growing abyss of doubt.
How did he go about it? For the scientists, Newton had been their long-awaited Savior. “God said let Newton be,” Alexander Pope gloriously exclaimed. Everyone was caught up in a physical mechanical explanation, the mathematical ways of the world. Everything can be reduced to cause and effect, which was the magical universal formula.
Kant was into absolute statements and shunned the relativist point of view of the humanists as well as the skeptical outlook. Truth should not be simply useful, it should be true and valid in all cases and without exceptions. So what did he do after he actually “woke up,” after Instant Karma hit him over the head, as John Lennon would say?
He spent all his time and energy to come up with a coherent theory, which would both fit into the scientific worldview, yet at the same keep idealism apart and intact. It was, so-to-speak, an updated version of what Plato intended to prove more than two millennia before Kant. The latter clothed Plato's theory in modern jargon and made it more resonant and consistent with our modern day beliefs.
In fact, Kant's idea in a nutshell is simple and straightforward. There are two kinds of events in the world. The one we mostly refer too is called phenomena. This can be equated with Plato's illusory world of shadows and appearances. All such physical events have an underlying cause. Natural laws, such as gravity, apply to these events at all times. So far, it seems to embrace a materialistic point of view, which would make Newton smile.
Yet then he adds that there is another set of events called noumena. These events are un-caused and separate from natural laws. They are how things really are ("das Ding an sich"), the Superior Reality that Plato describes when all is seen in the light of the illuminating sun.
An example of a noumenon would be free will. We are all free to decide and hence always responsible for our actions. According to Kant, it is not only what is observable, our behavior, that defines us, but more so, our mental representations and intentions; they actually reveal much more about us. There is also what he calls the Transcendental Ego, that part of us that would most resemble a soul. This is however different from an empirical ego, the storehouse of our sensations and mental contents; the Transcendental Ego has no content; it is pure thought and hence 100% noumenon.
In other words, yes, there is another plane of existence not one far away from us but actually within us. Science cannot delve into those mysteries because it lacks tools for it. Science can only observe and make laws about phenomena and that's as far as it can get. So the question whether God exists or not is not a valid scientific question. One cannot prove nor falsify the existence of God or a spiritual entity simply because these questions all belong to the world of noumena.
Some might not be convinced yet about the contributions to philosophy and even psychology that Kant's views bring. According to Kant, the mind is active in creating and organizing our worldview, a point of view that strongly influenced Jean Piaget. Our mind is not simply passively taking in information, yet it is quite involved in shaping our thoughts and our opinions. In fact, it is actually the objects that conform themselves to our understanding. These thoughts and impressions are later categorized in what Piaget later called "schemata.”
We can see that Kant rescued us from a dilemma that would have engulfed idealism once and for all. It is not saved yet, but there are many who try hard to make others see that there is more to human beings than all that is visible and scientifically and rationally explained. There is another part that simply defies logic, that knows no boundaries, that cannot be explained and tracked down by science and that is, after all, pure noumenon.