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

The maximum accommodation of a normal eye is reached when the object is at a distance of 25 cm from the eye. The focal length of the eye lens cannot be decreased below this minimum limit. Thus, an object placed closer than 25 cm (or very close to the eye) cannot be seen clearly with a normal eye.

Normal eye is unable to see clearly the objects placed closer than 25 cm due to power of accommodation of eye.  

Ciliary muscles in the eye are not contract near. Because of this reason, normal eye is not able to see clearly the objects placed closer than 25 cm.