Speaking to Metal Hammer, Black Sabbath guitarist Tony Iommi talked about how he wrote “Symptom of the Universe”.

Interviewer asked “Back when you were writing ‘Symptom of the Universe,’ which guitarists were you in competition with?”, Iommy responded:

“Me. I was in competition with myself. I would always try to come up with more and more inventive ideas – different tunings, changing the amps, just fiddling about with the guitars really.

I would constantly be trying to improve things and change things. I didn’t really listen to other people, just in case I started playing someone else’s riff by mistake.”

Interviewer said:

The second half of ‘Symptom of the Universe’ heads off into weird, prog-jazz territory. Was that you, deliberately setting out to write a prog-metal song?

Tony said:

“No, that was just us jamming together. It was something I came up with and Geezer [Butler] and Bill [Ward] followed, and then Ozzy came in.

But then a lot of Sabbath’s stuff went to places you didn’t think they would go. I remember when we had Ronnie Dio with us, I played him the track ‘Die Young.’

It went into this completely different tempo, and he went, ‘You can’t do that!’ I went, ‘Of course we can. It’s what we’ve always done.'”

According to Wikipedia, the main riff to “Symptom of the Universe” is considered to be a structural and sonic predecessor to thrash metal.

Click here to entire interview via Louder Sound.