I quite liked that and still do. I was aware of his larger works like Music in 12 Parts:
Which is roughly in the same style as the music from the same time of Steve Reich:
But for quite some time now, Philip Glass has been writing music that is at least partly indebted to the older instrumental forms like the symphony and concerto. He has an interesting way of thinking about them. Having worked a great deal in the realm of theater he thinks of certain kinds of music as having what he calls a "narrative base". In other words there is either some text that offers narrative clues or, in the case of a concerto, the solo instrument is itself a kind of narrator. But the pure symphonic form, what he calls "numbered symphonies" has no narrative base other than "the language of music and the lineage of symphonic writing". [from the liner notes to the recording of the Symphony No. 9 conducted by Dennis Russell Davies.]
It is interesting to me that at this point in his life Philip Glass is so interested in the "lineage of symphonic writing". He relates that he and other students used to spend a lot of time listening to Mahler and Bruckner symphonies when they were undergraduates at the University of Chicago. Steve Reich, on the other hand, seems to have had no interest at any time in this kind of music.
There is certainly a rich lineage of symphonic writing and particularly when it comes to a symphony labeled with the number "9" as this resonates with the last completed symphonies of Beethoven, Bruckner and Mahler. Glass says that he actually had the Mahler 9 more on his mind than the Beethoven 9.
Here is the first movement of the Symphony No. 9 by Philip Glass.
Any symphony with no text and no external references he calls a "numbered symphony" and the lineage is that of the great symphonic writing extending from Haydn right up to the present day, with these symphonies of Philip Glass.
The other category, that of what he calls "narrative" symphonies, would include several other symphonies by Glass, such as his Symphony No. 6 "Plutonian Ode" with text by Allen Ginsberg. Examples by other composer would include the Symphony No. 9 by Beethoven with text by Schiller, but also the Symphony No. 6 by Beethoven, the "Pastoral" because of its programmatic aspect given by the composer. Another like this would be the Symphonie fantastique by Berlioz with its program. I would put "La Mer" by Debussy in this category because of its obvious symphonic nature. There are a host of tone poems that you might argue fall in this category, but it would probably be better to allow them their own category to avoid confusion.
Just for fun, let's have a listen to an older member of this lineage and one with the label "9": the Symphony No. 9 by Joseph Haydn, from very early in his output: