It is based on an interpretation of history which regards men as slowly advancing--pedetemtim progredientes--in a definite and desirable direction, and infers that this progress will continue indefinitely.

And it implies that, as The issue of the earth's great business, a condition of general happiness will ultimately be enjoyed, which will justify the whole process of civilisation; for otherwise the direction would not be desirable.

There is also a further implication.

The process must be the necessary outcome of the psychical and social nature of man; it must not be at the mercy of any external will; otherwise there would be no guarantee of its continuance and its issue, and the idea of Progress would lapse into the idea of Providence.
As time is the very condition of the possibility of Progress, it is obvious that the idea would be valueless if there were any cogent reasons for supposing that the time at the disposal of humanity is likely to reach a limit in the near future.

If there were good cause for believing that the earth would be uninhabitable in A.D.2000 or 2100 the doctrine of Progress would lose its meaning and would automatically disappear.