Ultimately, note that Bertrand Russell's essay is a criticism against the liberal arts education, particularly as it was practiced by the Victorians. In that context, education was largely focused upon the memorization of the Classics (i.e., Greek and Roman literature). For Russell, studying the Classics and the Arts wasn't itself the problem; rather, the problem lay in the single-minded zeal by which these subjects were celebrated as the quintessential high-water mark of human achievement.

Ultimately, for Russell, this kind of singular focus amounted to an intellectual dead end. This is because once one focuses one's attention so intently upon the past, one stops looking toward the future. In this essay, when Russell is discussing science, he isn't speaking so much about scientific knowledge, but, more deeply, he is discussing the mindset and perspectives which underlie the entire scientific process. Science is progressive. It is always looking toward the future, always looking to advance the boundaries of what is understood. Present practitioners are always building upon the achievements of their predecessors, just as future generations will build upon the achievements of the present one. To Russell, this mindset is itself of extraordinary value, as a cultivator of human achievement.

Bertrand Russell states that.

One defect, however, does seem inherent in a purely classical education--namely, a too exclusive emphasis on the past (para. 5).

He goes on to explain that looking only toward the past blinds us to the present and the possibilities of the future.  What we see in the past is refined or perfected, but the present can be raw and unattractive, and looking at the past does not necessarily help us to see what is around us or to imagine what might be.  He points out that as we view the past, what we do not see is that the beauty and richness came out of the same raw and unattractive state, something that is often not addressed.  This leaves us with misperceptions about our own times and a failure of imagination about the future.