The contrast in the parable is one of the absurd, ridiculous comparisons we sometimes find in the parables. On one side is the speck of dust or wood, a splinter, irritating the eye. On the other side is a beam, a huge piece of wood that could be used for constructing the roof of a house. It is obviously absurd that somebody could go round with a beam in his eye, something hundreds of times bigger than the eye, and not be aware of it. Unless what is meant is such a comprehensive problem that it means the person is, in fact, blind while thinking that he can see.
It brings to mind the text of John 9, the healing of a man born blind. Towards the end of that chapter, Jesus says that he has come into the world so that the blind might see and that those who see might become blind. He is present in our world for judgement. (One of the points of today's parable is that judgement about others does not belong to us.) Those who know they are blind, or partially sighted, or who see but with something irritating their eyes, are in a happier situation than those who think they can see, think they see everything, clearly, and without any difficulty. This is what emerges in the reaction of his interlocutors in John 9: 'are you saying we are blind?' 'If you were blind', answered Jesus, 'you would be without fault, but because you say 'we see' your guilt remains.'
The parable today does not invite us to narcissistic and egocentric introspection. go look into yourself to try to identify the plank that's blocking your vision. First of all we don't need encouragement to be narcissistic and self-preoccupied, worried about our own spiritual difficulties. Secondly it seems as if we would be moving in a circle, trying to see things when there is a radical problem with our sight! The whole point of the parable seems to be that my blindness is so comprehensive that I will not be able to find the problem by myself.
So we must look to Christ which is, in any case, always the better and wiser thing to do. Earlier in the week we read from Paul's letter to the Colossians that 'in Christ are hidden all the treasures of wisdom and knowledge'. Now imagine this absurdity, of a person saying 'I have seen all the treasures of wisdom and knowledge, I know all the treasures of wisdom and knowledge'. It is clearly absurd for any human being to say this, as absurd as a man walking round with a plank sticking out of his eye. It can never be so. The treasures of wisdom and knowledge are infinite, so we remain always learners, always disciples. We never arrive at the height of our Master: it is another comment in today's gospel which is therefore calling us to docility, to being open always to learning more, to seeing afresh, waiting for our vision to be strengthened and clarified, for new things to be presented for our vision.
Paraphrasing Jesus' way of speaking we can say: how happy are you who have splinters in your eyes now because you know your need for help and you will see. But woe to you who think you can see now because all you are really seeing is a plank blocking true vision, confusing and distorting and darkening your vision of what is true. Turn towards Christ, then, like so many blind people in the gospel and say 'Lord, that I may see'.