Isolated System for Principle of Conservation of Energy

According to Physics Matters GEO 'O' Level Physics (4th edition) by C. Chew, S.F. Chow and B.T. Ho:

The Principle of Conservation of Energy states that the energy cannot be created or destroyed, but can be converted from one form to another. The total energy in an isolated system is constant.

But what's an isolated system? According to Fundamentals of Physics (10th edition) by D. Halliday, R. Resnick and J. Jewett:

… the system is isolated from its environment; that is, no external force from an object outside the system causes energy changes inside the system.

Let us now consider a free-falling object. We often use it to illustrate the principle of conservation of energy (e.g. KE + GPE = constant). But are we "allowed" to do so? After all, we would not consider a free-falling object to be an isolated system because weight is an external force acting on the object. But why does the principle of conservation of energy holds for a free-falling object?

Hint: Redefine the system you consider to include …