Maturity is having the courage to use one's own intelligence!

Natural science physics contains in itself synthetical judgments a priori, as principles. ... Space then is a necessary representation a priori, which serves for the foundation of all external intuitions.

There is a limit where the intellect fails and breaks down, and this limit is where the questions concerning God and freewill and immortality arise.

Act in such a way that you treat humanity, whether in your own person or in the person of any other, never merely as a means to an end, but always at the same time as an end.

Always regard every man as an end in himself, and never use him merely as a means to your ends [i.e., respect that each person has a life and purpose that is their own; do not treat people as objects to be exploited].

In the mere concept of one thing it cannot be found any character of its existence.

The business of philosophy is not to give rules, but to analyze the private judgments of common reason.