The quality of being able to be imagined, thought of, or believed to be possible.
From 'conceivable' (able to be conceived) plus the suffix '-ity' (denoting a state or quality). Traces back through 'conceive' to Latin 'concipere' (to take in, become pregnant with an idea).
Medieval philosophers debated the conceivability of God—if you can imagine something, does it help prove it exists? This word sits at the intersection of imagination and reality, which is where a lot of human thought lives.
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