One crucial premise in the argument from illusion is the Phenomenal Principle. It states that if there sensibly appears to be something that possesses a sensible quality, then there is something of which the subject is aware that has that sensible quality. The principle thus enables the inference from a mere appearance to an existence (usually a mental one). In the argument from appearance, a similar move is taken by some philosophers—they infer a content from a mere appearance. There are (...) two kinds of defences for the Phenomenal Principle in the literature, namely, the epistemological one (e.g. H.H. Price) and the semantic one (e.g. Frank Jackson). I argue that neither consolidates the Phenomenal Principle. I particularly demonstrate that the appearance verb in premise 1 of the argument from illusion is not used in the phenomenal sense as it is used in the Phenomenal Principle, which renders the argument essentially invalid. To avoid invalidity, the proponents either give up the phenomenal use, which makes the argument unable to serve its original purpose, i.e. inferring an unusual existence, or they insist on the phenomenal use in all premises of the argument, which will trivialise the argument. I also demonstrate that a similar objection applies to the argument from appearance. (shrink)