 A third approach to modal conditions on knowledge worthy of mention is the requirement that for a subject to know that p, she must rule out all “relevant alternatives” to p. Significant early proponents of this view include Stine 1976, Goldman 1976, and Dretske 1981. The idea behind this approach to knowledge is that for a subject to know that p, she must be able to “rule out” competing hypotheses to p—but that only some subset of all not-p possibilities are “relevant” for knowledge attributions. Consider for example, the differences between the several models that have been produced of Apple’s iPhone. To be able to know by sight that a particular phone is the 6S model, it is natural to suppose that one must be able to tell the difference between the iPhone 6S and the iPhone 7; the possibility that the phone in question is a newer model is a relevant alternative. But perhaps there are other possibilities in which the belief that there is an iPhone 6S is false that do not need to be ruled out—perhaps, for instance, the possibility that the phone is not an iPhone, but a Chinese knock-off, needn’t be considered. Likewise for the possibility that there is no phone at all, the phone-like appearances being the product of a Cartesian demon’s machinations. Notice that in these cases and many of the others that motivate the relevant-alternatives approach to knowledge, there is an intuitive sense in which the relevant alternatives tend to be more similar to actuality than irrelevant ones. As such, the relevant alternatives theory and safety-theoretic approaches are very similar, both in verdict and in spirit. As in the case of a safety theorist, the relevant alternatives theorist faces a challenge in attempting to articulate what determines which possibilities are relevant in a given  situation.[21]