From Lenin’s The Collapse of the Second International, part II (1915):
“To the Marxist it is indisputable that a revolution is impossible without a revolutionary situation; furthermore, it is not every revolutionary situation that leads to revolution. What, generally speaking, are the symptoms of a revolutionary situation? We shall certainly not be mistaken if we indicate the following three major symptoms: (1) when it is impossible for the ruling classes to maintain their rule without any change; when there is a crisis, in one form or another, among the “upper classes”, a crisis in the policy of the ruling class, leading to a fissure through which the discontent and indignation of the oppressed classes burst forth. For a revolution to take place, it is usually insufficient for “the lower classes not to want” to live in the old way; it is also necessary that “the upper classes should be unable” to live in the old way; (2) when the suffering and want of the oppressed classes have grown more acute than usual; (3) when, as a consequence of the above causes, there is a considerable increase in the activity of the masses, who uncomplainingly allow themselves to be robbed in “peace time”, but, in turbulent times, are drawn both by all the circumstances of the crisis and by the “upper classes” themselves into independent historical action.
Without these objective changes, which are independent of the will, not only of individual groups and parties but even of individual classes, a revolution, as a general rule, is impossible. The totality of all these objective changes is called a revolutionary situation. Such a situation existed in 1905 in Russia, and in all revolutionary periods in the West; it also existed in Germany in the sixties of the last century, and in Russia in 1859-61 and 1879-80, although no revolution occurred in these instances. Why was that? It was because it is not every revolutionary situation that gives rise to a revolution; revolution arises only out of a situation in which the above-mentioned objective changes are accompanied by a subjective change, namely, the ability of the revolutionary class to take revolutionary mass action strong enough to break (or dislocate) the old government, which never, not even in a period of crisis, “falls”, if it is not toppled over.”
The full text is at marxists.org.