Why is a normal eye not able to see clearly the objects placed closer than 25 cm?

The least distance for distinct vision for a normal human eye is 25 centimetres. Therefore, in order to see the printed matter in a book clearly, without straining our eyes, the book must be held at a distance of 25 centimetres from our eyes. When it is placed nearer to the eyes, the printed material in the book appears blurred and we feel strain in our eyes.