 These two assumptions are intended to ensure that the definition of computation that results is not too narrow. This is, it ensures that no computable function will fail to be Turing-computable solely because there is insufficient time or memory to complete the computation. It follows that there may be some Turing computable functions which may not be carried out by any existing computer, perhaps because no existing machine has sufficient memory to carry out the task. Some Turing computable functions may not ever be computable in practice, since they may require more memory than can be built using all of the (finite number of) atoms in the universe. If we moreover assume that a physical computer is a finite realization of the Turing machine, and so that the Turing machine functions as a good formal model for the computer, a result which shows that a function is not Turing computable is very strong, since it implies that no computer that we could ever build could carry out the computation. In Section 2.4, it is shown that there are functions which are not Turing-computable.