Substance is not more real than appearance, nor appearance more real than essence, but only differently real. When the word reality is used invidiously or eulogistically, it is merely in view of the special sort of reality which the speaker expects or desires to find in a particular instance. So when the starving gymnosophist takes a rope for a serpent, he misses the reality of that, which is lifeless matter......W hen substance is asserted, appearance is not denied ; its actuality is not diminished, but a significance is added to it which, as a bare datum, it could not have.(from Scepticism and Animal Faith)
The gymnosophists/naked sages known in India as ‘avadoothas’ or sky-clothed are generally far from starving. I’ve seen two myself, one basking on a pavement in Bangalore and the other marching along a country road in Andra Pradesh. The only kit they carry is a water pot made from a gourd and a strong staff. At the Kumba Meelah when they take their bath in the ganges en masse, films of this auspicious event show them to range from well-fed to corpulent.
The classical confusion of snake for rope occurs at dusk. Error happens as the result of a defect in the conditions of perception, the default is veridicality. The advaitic view is similar to Santayana’s (qv above) and marginal notes in Dasgupta’s History of Indian Philosophy from Santayana’s library show that he appreciated its insights:
It is because we have an awareness of blueness that we speak of having perceived a blue objectNote on page 154 of History of Indian Philosophy (taken from George Santayana’s Marginalia: A Critical Selection Bk.I ed. John McCormick)
How good all this is
The idea of the illusion having its locus in the substratum of the rope broadly conforms to Santayana’s concept of substance and the illusion itself has its link to reality through its counterpositive or a real snake.