"For the purpose of this treatise we may, perhaps, define it as the power to see what is hidden from ordinary physical sight. It will be as well to premise that it is very frequently (though by no means always) accompanied by what is called clairaudience, or the power to hear what would be inaudible to the ordinary physical ear; and we will for the nonce take our title as covering this faculty also, in order to avoid the clumsiness of perpetually using two long words where one will suffice."