Why and how did "a sensible boy" become "intelligent and prudent"?

Solution 1:

The English meaning of the word sensible seems to stem from a medieval theory of mind.

The word sensible is an adjective which in the 19th century had two different meanings, related to the two different nouns sense and sensibility:

sense: 3 [mass noun] a sane and realistic attitude to situations and problems

sensibility: 1 the quality of being able to appreciate and respond to complex emotional or aesthetic influences; sensitivity

The adjective sensible connected with the noun sensibility had essentially the same meaning as the Italian word sensibile; this meaning is now nearly obsolete.

These two meanings both stem from the original meaning of the noun sense:

sense: 1 a faculty by which the body perceives an external stimulus; one of the faculties of sight, smell, hearing, taste, and touch

How did sense evolve from meaning 1 to also encompass meaning 3? This seems to be related to the medieval theory of mind. From the OED, we have (with a first citation from 1398)

common sense 1. An ‘internal’ sense which was regarded as the common bond or centre of the five senses, in which the various impressions received were reduced to the unity of a common consciousness. Obs.

One of the citations they give is from Richard Burton's Anatomy of Melancholy (ca. 1630). This book lists five outward senses: sight, hearing, smelling, taste, and touching; and three inner senses: common sense, phantasie or imagination, and memory. It gives the following fairly clear explanation of common sense:

Inner Senses are three in number, so called because they are within the brain-pan, as common sense, phantasie, memory. Their objects are not only things present, but they perceive the sensible species of things to come, past, absent, such as were before in the sense. This common sense is the judge or moderator of the rest, by whom we discern all differences of objects; for by mine eye I do not know that I see, or by mine ear that I hear, but by my common sense, who judgeth of sounds and colours: they are but the organs to bring the species to be censured; so that all their objects are his, and all their offices are his.

This original meaning of common sense has now evolved to what we call sense, and gave rise to current meaning of sensible.