In this issue, Pila (2009) has criticised the recommendations made by requirements engineers involved in the design of a grid technology for the support of distributed readings of mammograms made by Jirotka et al. (2005). The disagreement between them turns on the notion of “biographical familiarity” and whether it can be a sound basis for trust for the performances of professionals such as radiologists. In the first two sections, this paper gives an interpretation of the position of each side in (...) this disagreement and their recommendation for the design of technology for distributed reading, and in the third the underlying reasons for this disagreement are discussed. It is argued that Pila, in attempting to make room for mistrust as well as trust, brings to the fore the question of having and reflecting upon reasons for trust or mistrust. Pila holds that biographical familiarity is not a sound reason for trust/mistrust, as it seems to obliterate the possibility of mistrust. In response to her proposal, an analysis is proposed of the forms of trust involved in biographical familiarity. In particular, implicit trust is focused upon—as a form of trust in advance of reasons, and as a form of trust contained (in the logical sense) within other reasons. It is proposed that implicit trust has an important role in establishing an intersubjectively shared world in which what counts as a reason for the acceptability of performances such as readings of X-rays is established. Implicit trust, therefore, is necessary for professionals to enter into a “space of reasons”. To insist upon judgements made in the absence of the form of implicit trust at play in biographical familiarity is to demand that radiologists (and other relevantly similar professionals) make judgements regarding whether to trust or mistrust on the basis of reasons capable of being reflected upon, but at the same time leave them without reasons upon which to reflect. (shrink)