The first line of Sipser book for the Chapter- 'Time complexity', says that:
Even when a problem is decidable and thus computationally solvable in principle, it may not be solvable in practice if the solution requires an inordinate amount of time or memory.
My question is, yes we do have limited space and time, so the above statement makes the whole P and PSPACE concept as vague, right? If it so, what are these problems? Are such huge inputs acceptable in practice?