In

Grundlegung der Metaphysik

der Sitten

， Kant determines the moral law as a synthetic proposition a priori and relates its possibility to a so-called “Third” in the third chapter of

Grundlegung

. There is， however， little agreement among scholars as to what such a “Third” actually is. According to the descriptions proposed by Kant about such a “Third”， based on the analysis of the theoret

ical task set for the third chapter of

Grundlegung

， and related to the conception of the Third in

Critique of Pure Reason as

well， the “Third” of Kant’s moral philosophy can be determined to be the autonomy， which then further explained as the “type” of

Critique of Practical Reason

. The problem of the “Third”， therefore， reveals Kant’s concern with how morality is realized in the phenomenal world where sensibility and understanding are correlated to each other， whose clarification help us to better understand the third chapter of the

Grundlegung

and even to better understand the entire moral philosophy in Kant.