The argument from illusion is an argument for the existence of sense-data.
When we observe a straight stick half submerged in water, it appears bent. This is an illusion. Seems the stick is not in fact bent, the bent stick that we are seeing is an illusion. It is examples like this that have led some philosophers to argue that what we see (the bent stick), is not what actually exists in reality (a straight stick).
Therefore, all we really perceive is said to be non-physical ’sense-data’. This is all we can be sure of. We do not know how much, if at all, this sense-data represents the real world. All that we are really aware of is this non-physical experience in our own personal theatre.
In other words, what we are perceiving is not the stick, but mere sense-datum of a stick.