"The most reliable and useful courage was that which arises from the fair estimation of the encountered peril, but an utterly fearless man is a far more dangerous comrade than a coward."
I like the practical Melville. In many ways this line just seems easy - courage is when you do something having taken into account all the dangers and weighed that against the benefits. Useful courage is a sound decision. Most of us like to think we fall into that category. However there are people who fear nothing. Some seem to be politicians with a name that rhymes with rump. Those fearless people, Melville writes, are as thoughtless as the cowards. Cowards fear. Fearless people fear nothing and therefore will do most anything without thought of danger or benefit. Ahab, like our current captain, is fearless. Spoiler alert - it tunrs out badly for his shipmates.