Seeing is not always believing. Presuppositions rule. Jesus heals people and the Pharisees claim he is doing it by the power of Satan. He raises Lazarus from the dead and they decide to kill Him. Their presuppositions said that since He was not one of them, this cannot be allowed.
In the story of Lazarus (a different Lazarus) and the rich man, Jesus has the rich man saying from hell that he wants someone to go tell his relatives the truth. The response? Even if someone should rise from the dead, they will not believe.
Every event which might claim to be a miracle is, in the last resort, something presented to our senses, something seen, heard, touched, smelled, or tasted. And our senses are not infallible.
If anything extraordinary seems to have happened, we can always say that we have been the victims of an illusion. If we hold a philosophy which excludes the supernatural, this is what we always shall say. What we learn from experience depends on the kind of philosophy we bring to experience.
Yet despite our fallible senses, miracles do occur. There have been stories about miracles throughout the ages, but Lewis draws a distinction between the fanciful stories and those that are recorded in Scripture:
The fitness of the Christian miracles and their difference from these mythological miracles, lies in the fact that they show invasion by a Power which is not alien.
They are what might be expected to happen when she is invaded not simply by a god, but by the God of Nature: by a Power which is outside her jurisdiction not as a foreigner but as a sovereign.
They proclaim that He who has come is not merely a king, but the King, her King and ours.
A miracle, then, is a friendly invasion. It is God refashioning His creation at a moment in time for a specific purpose. And we can believe what we see when we are submitted to the King.