The term 'THING-IN-ITSELF' is a philosophical concept first introduced by German philosopher Immanuel Kant IN his 1781 work, 'Critique of Pure Reason'. The term refers to the idea that objects and phenomena IN the world have an underlying reality that is independent of our perception of them. Kant argued that this underlying reality is unknowable and can only be experienced through intuition. The six-letter solution for this term is 'itself', which is a reference to the idea that the THING-IN-itself is self-contained and cannot be known or understood through any external means.