 At first glance, it is not at all clear how the premises are expected to support the conclusion. Here we should recall that the argument is probably intended to be perplexing and open to various interpretations, the better to confuse and mystify the audience. With this caveat in mind, one plausible reading is that ‘white horse’ names both the color and the shape of white horses, not only the shape. So ‘white horse’ names something different from what ‘horse’ names. Hence white horse, the extension of ‘white horse’, is not the same as (identical to) horse, the extension of ‘horse’.