Computation - or executing mathematical functions - is processing ideas held in human consciousness, so how can it exist naturally, outside that consciousness? Computers compute in a fundamentally same way as the abacus does - the move objects (electrons or beads) According to patterns imposed on them by the human mind which then interprets the outcomes of those movements as concepts (i.e. quantity.)
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Must fly so brevity is required.
Your criticism only applies if you limit the definition of computation to boolean logic, "mathematical functions" and symbol manipulation, narrowly explained.
To open things up a bit ask yourself, what does DNA do? Is it possible to enlarge our understanding of "computation" by thinking deeply about this?
Many quite serious people are beginning to say yes and I'm inclined to agree.
A large topic but I gotta go.
.d.