We live in a finite Universe. We have finite life times. We have finite amounts of space - whether it be our homes, our gardens or our hard drives. Thus, when we want to talk about the complexity of an algorithm, there are two natural choices. They are space and time.
When we talk of space and time, we often need to specify how we parametrise our measurement. For instance, the complexity may vary on the input itself, rather than the size of the input, when our input set .