Attentive readers of this blog will already be familiar with the distinguished name of The Manolo, and they will know that there is nothing from the shoes that The Manolo cannot tell you. Well, it happens now that he has been asked by one of his readers to pronounce on the philosophical question of what it is that defines the shoes, since the aforesaid reader and her husband disagree about the matter, he saying that 'the flip flop and slipper should be defined as a shoe as they cover the feet', and she demurring at this.
The Manolo for his part declares the husband to be right ('the slipper is the type of the shoe, as is the flip-flop'), but goes on to exclude the slipper and the flip-flop from being taken seriously nonetheless, as in any collection of the shoes they are of no consequence and indeed unworthy of mention.
The New Shorter Oxford may seem at first to support The Manolo's verdict, offering...
An outer foot-covering of leather, plastic, fabric, etc...This would apply to the slippers and (at least partially) the flip-flops. But the same definition continues...
... having a fairly stiff sole...And that may be thought to rule out much in the way of both the slippers and the flip-flops, in all the many cases where the soles are not sufficiently stiff - though the New Shorter Oxford does not specify how stiff 'fairly stiff' would have to be to count. Moreover, the definition given being merely the dictionary type of the definition, there are the further complications. For it is not essential to all the possible instances of the shoe that it be actually a foot-covering. The merely ornamental shoe is still meaningfully the shoe, is it not? So perhaps the answer The Manolo gives is most prudent when all is said and done, allowing him to accommodate the fact that the boundaries of the concept of the shoe are, as with many another concept, not altogether sharp.