Aristotle considers friendship the greatest external good, one integral to the attainment of happiness. However, while Aristotle limits distrust to what he calls imperfect forms of friendship, subsequent philosophers have stressed our uncertainty regarding the benevolence, beneficence and loyalty we may expect of friends. They do so in part because overcoming this uncertainty requires the exercise of the virtues of trust and loyalty if our friendships are to survive intact. For example, insofar as Aquinas holds that we cannot scrutinize the (...) wills of others - thus inviting uncertainty regarding their present and future conduct - he argues that friendship requires the virtue of hope as a cause of friendly love, a hope which helps us to make virtuous presumptions about others' wills. Likewise, Kant argues that all de facto friendships are plagued by epistemic uncertainty regarding the wills of others. In consequence, he treats loyalty as an unenforceable ideal of virtue. Kierkegaard goes further, framing his treatment of non-agapic love - in which he argues that friendship cannot be ethically justified - with a discussion of deception in Works of Love. If Aristotle is correct in thinking that friendship `is a virtue, or involves virtue', and that `loving is the virtue of friends', then addressing the epistemological, conceptual, and normative concerns these philosophers have regarding trust and loyalty between friends is needed to understand a central goal of the ethical life: the perfection of love. After a historical survey of the thought of these four thinkers regarding the relationship between friendship and loyalty, this study suggests that contemporary problems about the origins, nature, and limits of loyalty can be fruitfully resolved using insights derived from the historical survey. (shrink)