Schiller (1967):

In his Fourteenth letter, Schiller writes about the sense drive (sinnliche Treib) and the form drive (der Formtrieb) and the play drive (Spieltrieb) which is between the two:

3.  . . . . The sense-drive demands that there shall be change and that time shall have a content; the form-drive demands that time shall be annulled and that there shall be no change.  Tht drive, therefore, in which both the others work in concert (permit me for the time being, until I have justified the term, to call it the play-drive), the play-drive, therefore, would be directed towards annulling time within time, reconciling becoming with absolute being and change with identity . . . .

5. The sense-drive excludes from its subject all autonomy and freedom; the form-drive excludes from its subject all dependence, all passivity . . . . The play-drive, in consequence, as the one in which both the others act in concert, will exert upon the psyche at once a moral and a physical constraint; it will, therefore, since it annuls all contingency, annul all constraint too, and set man free both physically and morally.  When we embrace with passion someone who deserves our contempt, we are painfully aware of the compulsion of nature.  When we feel hostile toward another who compels our esteem, we are painfully aware of the compulsion of reason.  But once he has at the same time engaged our affection and won our esteem, then both the compulsion of feeling and the compulsion of reason disappear and we begin to love him, i.e., we begin to play with both our affection and our esteem. (p. 97)

In his Fifteenth letter, Schiller continues, as the commentary points out, linking the play drive “not only with human freedom but with human love” (p. 253):

2. The object of the sense-drive, expressed in a general concept, we call life, in the widest sense of this term: a concept designating all material being and all that is immediately present to the senses.  The object of the form-drive, expressed in a general concept, we call form, both in the figurative and in the literal sense of this word: a concept which includes all the formal qualities of things and all the relations of these to our thinking faculties.  The object of the play-drive, represented in a general schema, may therefore be called living-form: a concept serving to designate all the aesthetic qualities of phenomena and, in a word, what in the widest sense of the term we call beauty. (p. 105)

. . . . For to mince matters no longer, man only plays when he is the fullest sense of the word a human being, and he is only fully a human being when he plays (p. 107).