It's come to this --
**a theme is misplaced concreteness**
This is to say that a theme is a particularly memorable artifact of certain prolongations. This statement involves agreement (or complicity) with some Schenkerian ideas.
This proposes a musical value. To get there we have to consider dispensing with some habits, such as, **a theme is a good thing**. Misplaced concreteness sounds negative....; yes, just a wee bit.
A theme is a great thing. Your blue-haired old lady complains about "not enough tunes".
I agree with her. Sometimes the blue-haired ladies are correct.
Therefore, the misplaced concreteness that we hear as a theme must be continually re-purposed for specific prolongational ends.
Themes must be adaptable. In Bach suites the theme is adaptable, and does not always appear twice in the same form.
There is a counterpoint of reifications -- the theme is a reification; the prolongation is a different thing on a different speed and we reify it when we talk about it.
This is translated in first year music theory as "imperfect cadence", "half cadence", "perfect authentic cadence".
In post-tonal music these things are, except in Babbitt and and few others, considered as quaint a rhyme and verse, but I do not subscribe to this prevailing value.