Computation is the implementation of a symbol system.
Although the manipulation is based merely on the shape of the symbols, the system and its symbols must be systematically interpretable as having meaning. However, it was its main strength that became its down fall. Any attempt to define computation will always be and has to be the same. If cognition captures everything you can do formally and physically then why can't it capture what it is that minds are. The argument is extremely persuasive, especially as the way computation works seems to be the way certain aspects of our abilities work at an introspective level, for example, our logical reasoning capacities.