Although this essay is concerned primarily with Haydn's musical style, I must begin with a few remarks about his personality. In agreement with certain observers of his own time, such as the Swedish diplomat Fredrik Samuel Silverstolpe, I have for many years interpreted his personality in terms of a duality between earnestness and wit.1 Haydn was devout and dedicated to the Virgin, and especially in his late years a leading exponent of the musical sublime; at the same time he was often high-spirited, jovial, even joking. However, it must be remembered that wit connotes not only humor, but intelligence, inventiveness, and so forth; and that in the 18th century the concept humor itself was much broader than it is today, including for example the 'four humors' or temperaments: sanguine, choleric, melancholic, phlegmatic.2 Be that as it may, I have recently come to believe that to frame the matter solely in terms of this single binary opposition underplays a third, equally important aspect of Haydn's personality: his sensibility.3
The word "sensibility" has numerous different meanings, of which the relevant ones in this context are:4
 mental or emotional responsiveness toward the attitudes, feelings, or circumstances of others
 receptiveness to impression, acuteness of feeling
Closely related is "sensitivity," which often functions as a synonym for "sensibility" in senses  and  above, but also has the additional, negative meaning:
 quickness to take offense; touchiness
Both words are in turn related to "sentiment":
 tender, affectionate feeling
and to "sentimentality":
 affected, inappropriate, or excessive emotionality
All these words derive from sense (a capacity to appreciate or understand; recognition or perception through the senses or the intellect) and therefore ultimately from the Latin sensus, past participle of sentire, "to feel." (Just as in English these words overlap in meaning, so do the equivalent German words Empfindsamkeit, Empfindlichkeit, Sensibilität, Sentimentalität, although the correspondences between the two languages are not always consistent.) Thus in using "sensibility" in my title, I don't mean to exclude aspects of the concept that strictly speaking might better be expressed by one or more of these related words.
In my view, all of these aspects of sensibility were important in Haydn's personality, except No. 5, sentimentality. His receptiveness to impression (No. 2) may be taken for granted. His sensitivity (No. 3), especially to criticism, is now well understood; it often took exaggerated forms, such as his 'pre-emptive' rebuttal of negative reviews in the preface to the "Auenbrugger" keyboard sonatas Hob. XVI:35-39 & 20 (published 1780), in which he pointed out the similarity of the opening themes of two variation rondos.5 Perhaps his tendencies to loneliness and melancholy, and (late in life) his complaints of ill-health - even his unhappy marriage? - were related to hypersensitivity as well.
Less well understood, however, are the depth and range of Haydn's sensibility in the positive sense (Nos. 1 and 4). When outward circumstances permitted (primarily in Vienna and London), he enjoyed a rich and varied social and affectional life. The recipients of his affections included not only his lovers Luigia Polzelli and Rebecca Schroeter, but numerous intimate friends such as Maria Anna von Genzinger. His letter to Genzinger of February 1790 is often quoted, but primarily owing to his amusing description of his sudden removal from Vienna to Eszterháza, and his sorry state there; less often emphasized is his regret at being deprived of the social and artistic 'scene' of the capital:
Here I sit in my wasteland - forsaken ... almost without human contact - sad - full of the memory of past glorious days - yes, alas, past - and who knows when those pleasant days will come again? those wonderful parties? where an entire company is one heart, one soul - all those beautiful musical evenings - which can only be imagined, and not described - where all those inspirations? …