Perhaps the easiest way to understand the distinction between Keynes' and the Classicals' view is this. That subtle distinction ends up making a big difference. For a situation to be unstable, it must create conditions that cause at least one group to wish to change those conditions and that group must have the power to do so. To Classicals, when involuntary unemployment occurs, that group is potential workers. But they don' t necessarily actively seek it.
Potential workers are in a bind, however.
There are more willing workers than jobs, which means that someone will not get hired. It is in their best interests to do so if they want to be employed. Keynes didn't see it this way. Firms, to start, are already on the labor demand curve. Furthermore, it is quite possible that they might even resist lowering wages, even though there is the excess supply of labor.