In this passage, the line everyone knows is "Those who cannot remember the past are condemned to repeat it" but I think that the opening line might be the more insightful - you have to have an acknowledged position from which to start in order for progress to be made. Tradition and continuity is a prerequisite to progress.
Continuity necessary to progress.
Progress, far from consisting in change, depends on retentiveness. When change is absolute there remains no being to improve and no direction is set for possible improvement: and when experience is not retained, as among savages, infancy is perpetual. Those who cannot remember the past are condemned to repeat it. In the first stage of life the mind is frivolous and easily distracted; it misses progress by failing in consecutiveness and persistence. This is the condition of children and barbarians, in whom instinct has learned nothing from experience. In a second stage men are docile to events, plastic to new habits and suggestions, yet able to graft them on original instincts, which they thus bring to fuller satisfaction. This is the plane of manhood and true progress. Last comes a stage when retentiveness is exhausted and all that happens is at once forgotten; a vain, because unpractical, repetition of the past takes the place of plasticity and fertile readaptation. In a moving world readaptation is the price of longevity. The hard shell, far from protecting the vital principle, condemns it to die down slowly and be gradually chilled; immortality in such a case must have been secured earlier, by giving birth to a generation plastic to the contemporary world and able to retain its lessons. Thus old age is as forgetful as youth, and more incorrigible; it displays the same inattentiveness to conditions; its memory becomes self-repeating and degenerates into an instinctive reaction, like a bird's chirp.