Kant was interested in solving all of metaphysics' puzzle, with a focus on the controversy between the Rationalists and Empiricists.
The controversy was centered on the question of objective knowledge - could one know the world without experiencing it?
Rationalists believed that reason was the only tool for understanding the world, while Empiricists argued that experience was key.
Kant found that both were necessary - experience without reason was just meaningless, raw sensation.
Kantian philosophy is based on the belief that the world is understood through the perspective of a rational creature
This creature imposes "categories" of thought onto the world in order to understand it
The categories are not descriptive of the world as it really is, but they are universal in the sense that they can represent any object that can be experienced
As a result, there is a division between the world as it appears and the world as it really is
It is central to Kantian philosophy that the world as it really is completely unreachable for humanity
Kant was skeptical about the soul, but deduced that it had the faculty of understanding
Practical judgment is the faculty by which a person decides what to do, and is the source of an ethical life
If humans are to have an ethical life, they must first have free will
Kant established that humans have free will by distinguishing between the phenomenal and noumenal
Ethical living is living in accordance with pure practical reason
The categorical imperative is the fundamental principle of Kant's ethical theory
Kant's metaphysics and ethics are the most well-known aspects of his work
He used transcendental philosophy to understand the seemingly paradoxical nature of beauty
Kant was a classical liberal in tune with the spirit of the Enlightenment
He opposed democracy but also distrusted monarchy
He was an advocate of an ill-defined republicanism
Notes
Scruton has written several book on philosophy, including some other ones where he summarizes philsophies in a short treatment such as this.
Even summaries of Kant's work are dense. You need to take your time reading them. I made the mistake of trying to listen to this.
Kant is where so much of traditional philosophy places their arc, as to where philsophy really started to change in the dramatic fashion that it did.
A revisit of kant, or this summary, might be good after some experience with more modern analytical philosophy.
I've to read more about Kantian notions of free will, the noumenal and the phenomenal in light of AI and philosophy of the mind.