The degree of trust string physicists place in the their theory's viability is surprising in view of the theory's empirically unconfirmed and theoretically incomplete state. We argue that this trust is based on a web of partly implicit arguments which change the string physicists' assessment of the underdetermination of scientific theory building by the available empirical data. Scientists usually assume that the available empirical data could in principle be reproduced by various different scientific theories which may also predict different outcomes of future experiments. This "principle of scientific under- determination" normally prevents scientists from placing trust in a theory as long as its empirical predictions have not been confirmed to a substantial degree. String theorists, however, face a number of purely theoretical arguments at various levels which indicate a devaluation of scientific under-determination in the context of string physics. This devaluation increases the power of purely theoretical arguments in assessing a theory's viability. (see publication [1].)