Full Idea
Kant's philosophy is the contradiction of essence and existence; essence lies in the mind and existence in the senses; existence without essence is mere appearance, and essence without existence is mere idea.
Gist of Idea
For Kant, essence is mental and a mere idea, and existence is the senses and mere appearance
Source
report of Immanuel Kant (Critique of Pure Reason [1781]) by Ludwig Feuerbach - Principles of Philosophy of the Future §22
Book Reference
Feuerbach,Ludwig: 'Principles of the Philosophy of the Future', ed/tr. Vogel,M [Hackett 1986], p.35
A Reaction
The Sartrean challenge is that existence without essence is not mere appearance, but is the central feature of reality as it actually is. One might even flirt with the slogan 'existence is essence'.