Does a computer think if no one is around to see it?

I show that a computer cannot have unambiguous thoughts, not even about a number. What we believe computers do is our own convention. It may seem objective because we anchor it in the user interface. But many other conventions are possible, and they yield different computations, equally valid according to the principles of Computer Science. I prove that the alternative computations equally happen when a single computation is carried out, and in principle they can be accessed. I exemplify this with a program that computes the result for a given input, and then decodes it into the results for all other possible inputs. If thinking would be a computation, a computer would have different, possibly opposite thoughts, corresponding to many alternative computations it implements at the same time. I show probabilistically that the human mind does not have this ambiguity. Therefore, even if the human mind can be simulated by a computer, it cannot be reduced to computation.