At one time, the Buddha addressed the disciples thus: 'There is, O Bhikkhus, in the ocean a turtle, both of whose eyes are blind. He plunges into the water of the unfathomable ocean and swims about incessantly in any direction wherever his head may lead.
There is also in the ocean the yoke of a cart which is ceaselessly floating about on the surface of the water, and is carried away in all directions by tide, current and wind.
Thus these two go on throughout an incalculable space of time: perchance it happens that in the course of time the yoke arrives at the precise place and time where and when the turtle puts up his head, and yokes on to it.
Now, O Bhikkhus, is it possible that such a time might come as is said?'
"In ordinary truth, O Lord," replied the Bhikkhus, "it is impossible; but time being so spacious, and an aeon lasting so long, it may be admitted that perhaps at some time or other it might be possible for the two to yoke together, as said; if the blind tortoise lives long enough, and the yoke does not tend to rot and break up before such a coincidence comes to pass."
Then the Buddha said, "O Bhikkhus, the occurence of such a strange thing is not to be counted a difficult one; for there is still a greater, a harder, a hundred times, a thousand times more difficult than this lying hidden from your knowledge. And what is this? It is, O Bhikkhus, the obtaining of the opportunity of becoming a man again by a man who has expired and is reborn once in any of the four realms of misery.
"The occurrence of the yoking of the blind tortoise is not worth thinking of as a difficult occurrence in comparison therewith. Because those who perform good deeds and abstain from doing bad alone can obtain the existence of men and Devas.
"The beings in the four miserable worlds cannot discern what is virtuous and what vicious, what good and what bad, what moral and what immoral, what meritorious and what demeritorious, and consequently they live a life of immorality and demerit, tormenting one another with all their power.
"Those creatures of the Niraya and Peta abode in particular, live a very miserable life on account of punishments and torments which they experience with sorrow, pain and distress. Therefore, O Bhikkhus, the opportunity of being reborn in the abode of men is a hundred times, a thousand times harder to obtain than the encountering of the blind turtle with the yoke."
Explanation in terms of Identity existence in this world of Samsara.
A blind turtle in the ocean and a single place to yoke onto.
There you have it. With eyes blinded, unable discern what is virtuous and what vicious, what good and what bad, what moral and what immoral, what meritorious and what demeritorious, they will fail to find the truth within the Dharma although it be just an arm's distance away. With the slightest impediment even if you know that the lifesaving raft is available, even though you know its name and meditate without knowledge then you will not find it.