Gilead asked:

Skepticism of the existence of an external world, or the possibility of receiving artificial or false
perceptions, has been described by Hume and other philosophers. I think that such Ideas were
presented by ancient philosophers, such as Confucius and others.

My question is: How did these ancient philosophers visualize (if at all) mechanisms that will allow
such false perceptions to be received by a person?

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I wasn't aware that Confucius professed scepticism concerning the external world, but you are right
that philosophical scepticism has been around since ancient times.

The foundations for scepticism concerning the external world were laid by the Greek atomists. Here is
a fragment from the Presocratic philosopher Democritus (Kirk, Raven and Schofield The Presocratic
Philosophersp. 412):

But we in actuality grasp nothing for certain, but what shifts in accordance with the condition
of the body and of the things which enter it and press upon it.

What Democritus has seen is that ifall perception involves a chain of causes and effects, then no
inference from the way things appear to the way things really are can be certain. It is a familiar
observation that a bowl of lukewarm water feels hot if your hand is cold, and cold if your hand is hot.
Democritus means to draw the far more wide ranging conclusion that any perception involves the
causal effect of both the object perceived and the state of one's own body. Those two elements can
never in principle be factored out. We can never know how much of what we seem to perceive is due
to the object and how much is due to us.

According to Democritean atomism, the experience of warmth, or of colour, smell or sound, is illusory.
In reality, all that exists are atoms whirling in space. The seeming richness of human experience is
merely the effect of the physical interaction between our sense organs and the external world.
Democritus never doubts, however, that there isan external world. How could he? He is merely
claiming that we cannot know for certain how things are in the external world because all knowledge
relies on chains of cause and effect. In addition, our senses deceive us into thinking that there are
such things as warmth and red when in reality these 'experiences' are nothing but atoms knocking
into one another.

After Aristotle, the spectre of scepticism returned with the fearsome Pyrrho, who preached the gentle
doctrine that we should 'follow nature' rather than strive to determine whether or not our naturally
acquired beliefs are true, since every set of reasons in favour of a belief can be countered by equally
strong reasons against.

To my knowledge, neither Democritus or Pyrrho go so far as to postulate hypothetical mechanisms
that would explain how, e.g. my belief that there is a table in front of me might be false. This is the
most significant difference between Ancient scepticism, and the far more radical scepticism explored
by Descartes in his Meditations,with the aid of the hypothesis of an 'evil demon' who deceives me
into believing that there exists a world of objects in space, while in reality all that exists are states of
my own consciousness and their non-physical cause.

Geoffrey Klempner