Nature was at one time considered to be completely predictable. Newton's law of motion dictated that if, at a given time, the position and velocity of an object were known, then the object's position and velocity could be determined for any point in its past and any point in its future. This is what our intuition tells us too. But, as new discoveries continue to tell us, our intuition is merely a construct built up of misconceived notions about the world. Quantum Mechanics has shown us that nature is in fact inherently probabilistic; that regardless of our ability to build finely tuned measuring devices, it is impossible to ever know with complete certainty all of the information about an object, and thus to predict its future or extrapolate its past.
The idea of "probability" here is being used in a way that is different from our everyday idea of probability. A short passage from Stephen Hawking's The Grand Design explains this difference beautifully:
"It is important to realize that probabilities in quantum physics are not like probabilities in Newtonian physics, or in everyday life. We can understand this by [considering] . . . the pattern of holes built up by players aiming for the bull's-eye on a dartboard . . . the chances of a dart landing near the center are greatest, and diminish as you go farther out . . . any given dart can land anywhere, and over time a pattern of holes that reflects the underlying probabilities will emerge. In everyday life we might reflect that situation by saying that a dart has a certain probability of landing in various spots; but if we say that . . . it is only because our knowledge of the conditions of its launch is incomplete. We could improve our description if we knew exactly the manner in which the player released the dart, its angle, spin, velocity, and so forth. In principle, then, we could predict where the dart will land with a precision as great as we desire. Our use of probabilistic terms to describe the outcome of events in everyday life is therefore a reflection not of the intrinsic nature of the process but only of our ignorance of certain aspects of it. Probabilities in quantum theories are different. They reflect a fundamental randomness in nature."
The origin of this randomness is the "Uncertainty Principle", which states, among other things, that it is impossible to know both the position and velocity of an object simultaneously with complete precision. More on this later.
and people used to say thinking about the vastness of the universe makes an individual seem insignificant... now science is almost making an individual, as we know them, seem unreal. all i know is the more i understand the nature of the fundamentals of our physical reality, the more i feel blessed to exist as i do in this here and now
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