“ Wherefore just as timidity is opposed to fortitude by excess of fear, in so far as a man fears what he ought not, and as he ought not, so too fearlessness is opposed thereto by deficiency of fear, in so far as a man fears not what he ought to fear. ”
 Thomas Aquinas, Summa Theologica (1274). copy citation

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“Hence it belongs to fortitude that man should moderate his fear according to reason, namely that he should fear what he ought, and when he ought, and so forth. Now this mode of reason may be corrupted either by excess or by deficiency. Wherefore just as timidity is opposed to fortitude by excess of fear, in so far as a man fears what he ought not, and as he ought not, so too fearlessness is opposed thereto by deficiency of fear, in so far as a man fears not what he ought to fear. Reply Obj. 1: The act of fortitude is to endure death without fear, and to be aggressive, not anyhow, but according to reason: this the fearless man does not do.
Reply Obj. 2: Fearlessness by its specific nature corrupts the mean of fortitude, wherefore it is opposed to fortitude directly.” source