Kant created a new perspective in philosophy which had widespread influences on philosophy continuing through to the 21st century. He published important works on epistemology, as well as works relevant to religion, law, and history. One of his most prominent works is the Critique of Pure Reason, an investigation into the limitations and structure of reason itself. It encompasses an attack on traditional metaphysics and epistemology, and highlights Kant's own contribution to these areas. The other main works of his maturity are the Critique of Practical Reason, which concentrates on ethics, and the Critique of Judgment, which investigates aesthetics and teleology.

Kant suggested that metaphysics can be reformed through epistemology. He suggested that by understanding the sources and limits of human knowledge we can ask fruitful metaphysical questions. He asked if an object can be known to have certain properties prior to the experience of that object. He concluded that all objects about which the mind can think must conform to its manner of thought. Therefore if the mind can think only in terms of causality – which he concluded that it does – then we can know prior to experiencing them that all objects we experience must either be a cause or an effect. However, it follows from this that it is possible that there are objects of such nature which the mind cannot think, and so the principle of causality, for instance, cannot be applied outside of experience: hence we cannot know, for example, whether the world always existed or if it had a cause. And so the grand questions of speculative metaphysics cannot be answered by the human mind, but the sciences are firmly grounded in laws of the mind. Research on the structure and development of the brain in animals supports Kant's theory, at least for the perception of space; that is, spatial representation of the environment includes an innate component that predates any actual perception of the environment itself.

Kant believed himself to be creating a compromise between the empiricists and the rationalists. The empiricists believed that knowledge is acquired through experience alone, but the rationalists maintained that such knowledge is open to Cartesian doubt and that reason alone provides us with knowledge. Kant argues, however, that using reason without applying it to experience will only lead to illusions, while experience will be purely subjective without first being subsumed under pure reason.

Kant’s thought was very influential in Germany during his lifetime, moving philosophy beyond the debate between the rationalists and empiricists. The philosophers Fichte, Schelling, Hegel and Schopenhauer each saw themselves as correcting and expanding the Kantian system, thus bringing about various forms of German idealism. Kant continues to be a major influence on philosophy, influencing both analytic and continental philosophy.

Immanuel Kant was born in 1724 in Königsberg, the capital of Prussia at that time, today the city of Kaliningrad in the Russian exclave of Kaliningrad Oblast. He was the fourth of eleven children (four of them reached adulthood). Baptized 'Emanuel', he changed his name to 'Immanuel' after learning Hebrew. In his entire life, he never traveled more than a hundred miles from Königsberg. His father, Johann Georg Kant (1682–1746), was a German harnessmaker from Memel, at the time Prussia's most northeastern city (now Klaipėda, Lithuania). His mother, Regina Dorothea Reuter (1697–1737), was born in Nuremberg. Kant's grandfather had emigrated from Scotland to East Prussia, and his father still spelled their family name "Cant." In his youth, Kant was a solid, albeit unspectacular, student. He was reared in a Pietist household that stressed intense religious devotion, personal humility, and a literal interpretation of the Bible. Consequently, Kant received a stern education – strict, punitive, and disciplinary – that preferred Latin and religious instruction over mathematics and science.

Many of the common myths concerning Kant's personal mannerisms are enumerated, explained, and refuted in Goldthwait's introduction to his translation of Observations on the Feeling of the Beautiful and Sublime. It is often held that Kant lived a very strict and predictable life, leading to the oft-repeated story that neighbors would set their clocks by his daily walks. He never married, but didn't seem to lack a rewarding social life - he was a popular teacher and a modestly successful author even before starting on his major philosophical works.

Kant showed a great aptitude to study at an early age. He was first sent to Collegium Fredericianum and then enrolled at the University of Königsberg (where he would spend his entire career) in 1740, at the age of 16. He studied the philosophy of Leibniz and Wolff under Martin Knutzen, a rationalist who was also familiar with developments in British philosophy and science and who introduced Kant to the new mathematical physics of Newton. Knutzen dissuaded Kant from the theory of pre-established harmony, which he regarded as "the pillow for the lazy mind". He also dissuaded the young scholar from idealism, which was negatively regarded by most philosophers in the 18th century (The theory of transcendental idealism that Kant developed in the "Critique of Pure Reason" is not traditional idealism, i.e. the idea that reality is purely mental. In fact, Kant produced arguments against traditional idealism in the second part of the "Critique of Pure Reason"). His father's stroke and subsequent death in 1746 interrupted his studies. Kant became a private tutor in the smaller towns surrounding Königsberg, but continued his scholarly research. 1749 saw the publication of his first philosophical work, Thoughts on the True Estimation of Living Forces.

Kant is best known for his transcendental idealist philosophy that time and space are not materially real but merely the ideal a priori condition of our internal intuition. Also, he made an important astronomical discovery, namely the discovery of the retardation of the rotation of the Earth, for which he won the Berlin Academy Prize in 1754. Even more importantly, from this Kant concluded that time is not a thing in itself determined from experience, objects, motion, and change, but rather an unavoidable framework of the human mind that preconditions possible experience.[citation needed]

According to Lord Kelvin:

Kant pointed out in the middle of last century, what had not previously been discovered by mathematicians or physical astronomers, that the frictional resistance against tidal currents on the earth's surface must cause a diminution of the earth's rotational speed. This immense discovery in Natural Philosophy seems to have attracted little attention,--indeed to have passed quite unnoticed, --among mathematicians, and astronomers, and naturalists, until about 1840, when the doctrine of energy began to be taken to heart.