 The first reason is that it is extremely difficult to make any concrete predictions in these theories. String theory, in particular, is plagued by a lack of experimentally testable predictions because of the tremendous number of distinct ground or vacuum states in the theory, with an absence of guiding principles for singling out the physically significant ones (including our own). Though the string community prides itself on the dearth of free parameters in the theory (in contrast to the nineteen or so free parameters found in the standard model of particle physics), the problem arguably resurfaces in the huge number of vacua associated with different compactifications of the nine space dimensions to the three we observe. These vacua are either viewed as distinct string theories, or else as solutions of one and the same theory (though some deeper, unknown theory, as mentioned above). Attempts to explain why we live in the particular vacuum that we do have recently given rise to appeals to the infamous anthropic principle (Susskind, 2003), whereby the existence of humans (or observers) is invoked to, in some sense, “explain” the fact that we find ourselves in a particular world by restricting the possible ground states to those that could support such creatures in which we should expect our universe’s observed features to be typical. (See Weinstein, 2006, for a philosophical discussion of the usage of anthropic reasoning in string theory, including an ambiguity in the meaning of ‘typicality’ in this context; Azhar, 2013, further develops this discussion.)