One might easily ask what more is there for John Yolton to tell us about John Locke? In numerous works Yolton has revealed the varied aspects of Locke's philosophy and influence. And yet, as this admirable new study demonstrates, there remains much to investigate in Locke. Taking the opposite side of the philosophical coin to that found in Thinking-Matter (Minneapolis, 1983) with its account of eighteenth-century materialism, The Two Intellectual Worlds of John Locke addresses Locke's profound and hitherto neglected belief in spirits and the role that such immaterial entities played in the Essay Concerning Human Understanding.

Yolton confesses that he and other Locke scholars had been blind to the numerous references in the Essay to spirits. This is indeed an oversight because Locke located humanity within a universe that was home to both our material bodies and different species of spirits. The boundary between the material and immaterial is demarcated by human beings who have within themselves various immaterial aspects that must be located among the other spirits. Yolton then defines what Locke meant by “person” and its relation to “man” and “self.” “Man” is not born a “person” but may become one. Yolton proposes that “person” is best characterized “as a moral self,” an indication of righteous, and is paired with the secular nature of the “self.” Thus, the moral aspect of humanity (a faculty greatly important for the afterlife because it is “the locus of responsibility” [152]) is spiritual and cannot be material despite past and present readings to the contrary.

Having defined the key concepts of his study, Yolton reflects on Locke's characterization of man's environments. There are three: the universe itself; the domain of God, angels, and spirits; and the intellectual world comprising the field of human knowledge and understanding. Man exists within a tiny portion of the universe inside of which is our intellectual world built upon visible bodies and sensation. Our natural philosophy is constructed upon experience gained through examination of this intellectual world. However, Yolton reads Locke as not being content with this description. Without the inclusion of spirits in natural philosophy, Locke concludes that the discipline will become “lame and defective” (58). By modifying the subject matter of natural philosophy to incorporate the study of spirits, which are infinite in number and occupy the greater part of an intellectual world hidden from us, Locke suggests that our minds will become enlarged. Although we cannot know the spiritual realm with certainty since knowledge about it is beyond the faculties used in the material realm of the intellectual world, Locke encourages the effort because “the knowledge we have is sufficient to lead us to admire” God (75). Yolton makes much of Locke's broad sense of natural philosophy and concludes that it set Locke apart from his contemporaries. I will return to this shortly.

Next Yolton addresses what ideas may be held about spirits. This was no trivial question for early modern thinkers (Locke included) because the spiritual world of angels and God was the “hoped for destination of the virtuous and just men” (90), that is, “men” who had become “persons.” When considering this realm, Locke seems to abandon the epistemological method for which he is famous. Knowledge of spirits is described as predicated upon “opinion” and “possibilities.” Nonetheless, Yolton reveals how Locke seamlessly includes spirits and angels in his account of ideas and the faculties of the human mind through a depiction of humans as body and spirit. We occupy two locations on the Chain of Being: our material body is relatively low on the Chain while our spiritual soul is much higher but still lower than angels, for example. Thus, to fully understand humanity, one must consider the spiritual realm to which man partially belongs and hopes to live in the next life. For Yolton, Locke's strong commitment to the belief in spirits and the afterlife proves that early modern critics badly misread Locke.

Some readers may crave more historical context for Locke's views. Throughout the book Yolton makes frequent references to how Locke differed from his contemporaries with regard to the role played by spirits in natural philosophy, his focus on the Chain of Being, and the extent to which knowledge might be gained from the practice of natural philosophy. Dutch philosophers are mentioned, but British ones make only fleeting appearances. This is explained by Yolton's engagement with Andrew Fix's studies of Dutch thinkers who rejected the immediate presence of spirits in this world. While Yolton is surely right to insist upon the long-lasting belief in spiritual beings extending even into the supposed secular Enlightenment, his focus on Fix neglects more local context for Locke's views. The Cambridge Platonist Henry More, for example, insisted that spirits be brought under the umbrella of natural philosophy for reasons matching Locke’s. An important precedent is found in Robert Boyle who, as Jan Wojcik has demonstrated, argued that absolute certain explanations of natural and spiritual phenomena, which God had created according to divine understanding, were beyond our intellectual capacity. For Boyle, whom Yolton sees as exemplifying the search for empirical truth, reasoned hypotheses were the best our limited intellect could contribute to the pursuit of natural philosophical knowledge. Both More and Boyle may have affected Locke's account of spirits because his library is known to have contained books by both men. This relatively minor criticism aside, The Two Intellectual Worlds of John Locke is an excellent and thought-provoking book that gives not only Locke scholars but all historians and philosophers working on the intellectual climate of the seventeenth century much to consider.