The most-known work of Kant is the book "Critique of Pure Reason" ("Kritik der reinen Vernunft") that Kant published in 1781. Kant called his way of thought "critique", not philosophy. Kant said that critique was a preparation for establishment of real philosophy. According to Kant, people should know what human reason can do and which limits it has. In the "Critique of Pure Reason" Kant wrote several limits of human reason, to both feeling and thinking something. For sensation, there are two limits inside of human perception: space and time. There are no physical objects, but the limitations of our mind that work whenever we feel something through our senses. For thinking, he said there are twelve categories or pure rational concepts, divided into four fields: quantity, quality, relation and modality. Kant thought human reason applied those ideas to everything.