The title is meant to indicate that consciousness is being examined largely within the history of philosophy, and within the period of time from Descartes to Ayer. 
Investigators aiming to understand consciousness and minds usually try to take account of all individual human minds, so as to have the most data for  the most encompassing induction.  The problem with that approach is that because of the vastness of the data, its results tend to be vague, lacking the specificity of studies of individuals.  On the other hand, the problem with studies of individuals is that they cannot guarantee generality, as the opposing method can.  
This book's distinctive approach aims at a middle way, getting the best of the two opposing methods by drawing its data from the history of philosophy, especially the history of the great philosophers.