To Jim's point, self-taught usually doesn't mean "theory free".
Lately, very talented and music-degree-holding friends, as well as the internet, have held up Joni Mitchell as an example of a very successful songwriter and performer that had almost no relationship to musical pedagogy of any kind through most of their career... yet here's an excerpt from an interview she gave in 1996:
So how does Mitchell discover the tunings and fingerings that create these expansive harmonies? Here’s how she described the process: “You’re twiddling and you find the tuning. Now the left hand has to learn where the chords are, because it’s a whole new ballpark, right? So you’re groping around, looking for where the chords are, using very simple shapes. Put it in a tuning and you’ve got four chords immediately—open, barre five, barre seven, and you higher octave, like half fingering on the 12th. Then you’ve got to find where the interesting colors are—that’s the exciting part.
My Point? Well, I have a few:
- The fact that (in many parts of the music instrument/music tech world) we refer to so many different aspects of music education as "theory" does music education a huge disservice. Do we call knowing the difference between a verb, adverb, conjunction and pronoun "Language Theory"? No, we call it "learning English" or "basic grammar". One doesn't need to know the names of those concepts to say pretty and meaningful things, but would you openly opine as to how important it is to know them... or just look it up, commit it to memory, and move on?
**EDIT: see Jim's good points below re: taxonomic and epistemological concerns... I'm not trying to debate what is/isn't a theory, per the definition of the term, though I seem to keep doing that over and over... it's just that the word "theory" is usually reserved for more advanced applications in most other fields, yet in the music tech world, as of late, a plugin that can do something a simple as transpose a loop to a different key is billed as "no music theory needed"... "theory" has become synonymous with every aspect of musical communication... and, right or not, seems to be allowing ppl to categorize all forms of musical knowledge as something you can choose to know or not know, instead of treating it as basic table-stakes knowledge for effective performance and communication. A book on tape is not billed as "No English theory needed"... nobody wants to apply the "L" word... "literacy"... a good portion of what we label "music theory" overlaps with basic music literacy.
When people ask me about learning theory (shocking that they do, given my lack of musical gifts, but they do) I advise them to first learn all the things that aren't really theoretical at all**, IMHO, but just vocabulary/vernacular facts that will help you in any possible discussion with another musician... the basic table-stakes of western musical language. Learn the underlying patterns of intervals that make up the major and natural minor scales and all the modes of the major scale. Learn to construct those scales starting on any note. Learn what it means to be in a key, and how to determine what key you're likely in. Learn how to count in different time signatures. Learn the names and underlying constructions of all the chords diatonic to a given scale, learn to add tension notes to those chords and what you call them when you add those tensions... better yet, learn to do this both on your instrument, and in written/notated form.
None of the above really fits my definition of "theoretical"**, anymore than calling a verb a verb is theoretical.
Once you get into things like functional harmony and proposed rules for voice leading and counterpoint... and especially when you start seeing Hugo Reimann and Heinrich Schenker's names pop up, then you're getting into what I'd consider actual theoretical** work: concepts academics have put forth to try and explain why music works the way it does and what non-obvious rules might underly it's creation. How valuable this level of academic study is to the quality of your musical thinking is more debatable... it's no longer accepted and/or relevant fact in every possible music circle... I dig it, but I don't lump it in with the table stakes.
Getting back to the Joni Mitchell example, even someone naturally gifted enough AND self-confident enough to barrel through a music career without relying on a whole lot of traditional music education** ... is still going to pick up the table-stakes vocabulary after years of working with other musicians (I hear there are some good stories out there re: Jaco Pastorius learning to translate's Joni's own internally-derived music vocabulary for the rest of the band) . So much of this stuff is unavoidable... so just embrace it and learn to describe the basics of what you create to others with the shared vocabulary we've developed over hundred of years.
If all of this is 100% obvious to everyone, and we're just talking about the different theories of functional harmony, etc, apologies. But in my experience, when someone asks if they need to know theory, "WTF is a Maj7th chord?" is lumped in with far more debatable theoretical observations.