	"Psychology from an Empirical Stanpoint" is Brentano's best-known book, and much has been written about its "intentionality passage", quoted above. Brentano reintroduced the concept of intentionality into the philosophy of mind. However, in "Sexual Desire" (1986) the philosopher Roger Scruton describes the intentionality passage of "Psychology from an Empirical Standpoint" as both obscure and hesitant. Scruton believes that the obscurity of the passage is "compounded by Brentano's description of intentionality as the mark which distinguishes mental "phenomena" from physical "phenomena", the latter being described, not as objective features of the natural world, but as appearances." According to Scruton, while in later editions of "Psychology from an Empirical Standpoint" Brentano described intentionality as a property of mental activity, and characterized it as a kind of "mental reference", Brentano never makes clear precisely what kind of property he believes it to be anywhere in his writings.