necessary

(usually with the definite article) A place to do the "necessary" business of urination and defecation: an outhouse or lavatory.

Required, essential, whether logically inescapable or needed in order to achieve a desired result or avoid some penalty.

"Although I wished to think that all was false, it was yet necessary that I, who thus thought, must in some sense exist."

Unavoidable, inevitable.

"If it is absolutely necessary to use public computers, you should plan ahead and forward your e-mail to a temporary, disposable account."

Similar: inevitable, natural

Determined, involuntary: acting from compulsion rather than free will.