Given a Universe abiding solely by deterministic laws, Laplace's reasoning seems to make perfect sense. So let us take this deterministic Universe and imagine within it a scientist to whom the, let us call it: the ultimate formula, was made accessible. So, being capable to plug this hypothetical formula into a supercomputer, all the past states of the Universe as well as the future states would be absolutely and precisely knowable to the scientist.We may regard the present state of the universe as the effect of its past and the cause of its future. An intellect which at a certain moment would know all forces that set nature in motion, and all positions of all items of which nature is composed, if this intellect were also vast enough to submit these data to analysis, it would embrace in a single formula the movements of the greatest bodies of the universe and those of the tiniest atom; for such an intellect nothing would be uncertain and the future just like the past would be present before its eyes.
—Pierre Simon Laplace, A Philosophical Essay on Probabilities
So here's where the paradox arises: imagine the scientist uses said supercomputer to know how the next minute will play out: literally see the future. When this future is made visible to the scientist, then he is, therefore, able to act in a way that will create a different future than the formula predicted, rendering it useless in that sense.