he said on the preface of "history of western philosophy": "the student of Rousseau may have difficulty in doing justice to his connection with the Sparta of Plato and Plutarch." what does this mean? to check the full preface: https://somacles.wordpress.com/2009/06/10/preface-and-introduction-from-bertrand-russell%C2%B4s-history-of-western-philosophy/
Note: This answer is quite similar to Mauro's, but adds some explanation of the specific details of the quote.
The context is Russell's apology for knowing less about the specifics of any one philosopher or philosophical era than a specialist, and therefore treating each philosopher with less depth and focus than the specialist would prefer.
The highlighted quote is a justification for the utility of a single-author general survey, even despite the above reservations: The generalist author is better able to draw connections between widely separated philosophies than are the specialists, either working alone or together.
The quote gives the specific example of a connection between the French Enlightenment era philosopher Rousseau and the philosophy of Sparta as described by Plato and Plutarch as one that a specialist in either Rousseau or Sparta might easily miss.
It is an apology for the single-author history compared to history written by many specialists.
It means that the specialist of Rousseau may not be versed in the history of Ancient Greece, as well as the historian of Ancient Greece may not know about Hobbes and Lenin.
one of the purposes of [Russell's book] is to bring out such relations.