Quote:
ar·bi·trar·y
adj.
Determined by chance, whim, or impulse, and not by necessity, reason, or principle: stopped at the first motel we passed, an arbitrary choice.
Based on or subject to individual judgment or preference: The diet imposes overall calorie limits, but daily menus are arbitrary.
Established by a court or judge rather than by a specific law or statute: an arbitrary penalty.
Not limited by law; despotic: the arbitrary rule of a dictator.
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Quote:
nec·es·sar·y
adj.
Absolutely essential. See Synonyms at indispensable.
Needed to achieve a certain result or effect; requisite: the necessary tools.
Unavoidably determined by prior conditions or circumstances; inevitable: the necessary results of overindulgence.
Logically inevitable.
Required by obligation, compulsion, or convention: made the necessary apologies.
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These two words have different meanings.
In other words, you can't have a decision that is determined by "chance, whim or impulse" that
at the same time is "absolutely essential."
That would be a contradiction. No such word exists.
By definition, something
can't be arbitrary but necessary. It is either one or the other.
If you are looking for a word to describe a choice that
seems arbitrary, but is
in fact necessary, you could use some of the following suggestions.
justifiable
understandable
defensible
essential
Mr Mephisto