This is explained in the following extract from The Human Species (1961) Anthony Barnett:

An important discussion of this problem, published in Germany, in 1932, was based on studies of what happened when persons, blind from birth, had their sight restored by operation. Later, research was done on laboratory animals.

It had been assumed in the past that human beings (and other animals) have an innate ability to distinguish at least simple shapes, for instance squares from circles. In an adult, or even in quite a young child, this distinction is made ‘instantaneously' and without effort. This assumption has been shown to be false.

An adult blind from birth owing to cataract, and successfully treated so as to restore vision, reports at first only a confusing mass of lights and colours. No object however familiar from past experience of touching, smelling, and so on, can be identified by sight. Shapes are not identified at all: to distinguish a square or triangle from a circle the subject has to learn laboriously to count the corners, and what is learnt on one day is forgotten by the next. If the names of colours have been learnt, and the subject is shown an orange, he may at once name the colour, but he will not identify the object as an orange until he has touched it.

It was pointed out earlier that we recognise the things we see (or hear) even though the exact appearance they present varies according to the distance, the angle at which we see them, and so on. This perceptual generalization is quite absent in those learning to see. In one instance a man who had learnt to name a square, made of white cardboard, failed to identify it when the other side, painted yellow, was shown. Objects, of which the names have been learnt in one set of conditions, are not identified when seen against a different background or in light of a different colour. Such confusions, though never made by a normal adult, have nevertheless a counterpart in adult behaviour: for instance, ‘all Chinese look alike' – until we have had some experience of meeting different Chinese; or, all small brown birds look like sparrows – until we have had some experience of bird watching.

Eventually, with great persistence and after months or years, the congenitally blind person who has acquired vision may learn to make fairly full use of his eyes and even to read, but it is a painful experience. (pp. 120-121)

I couldn't work out what I was looking at because my brain had forgotten how to interpret what I was seeing. I couldn't even piece the features of faces together at first – the eyes looked as if they were on the top of people's heads.  (p. 24)

Jesus reportedly:

"...spit on his eyes and laid his hands upon him."

"Do you see anything?"

However, the former blind man couldn't distinguish what he was seeing.  He said:

"I see men; but they look like trees, walking." (8:24)

Jesus – according to the account – then:

"…laid his hands upon him; and he looked intently and was restored, and saw everything clearly." (6:25)

What at first seemed to be a report of one miracle is therefore really a report of two miracles.