The problem of whether Humans have free will or whether all our actions are pre-determined and our apparent free will is simply an illusion is profoundly important to Humanity, for the answer to this question will tell us whether we can determine our own future, and whether the concept of Morality actually exists (for if we had no free will, then there could be no such thing as morality!).
The solution is quite simple though and can be easily understood with the aid of the following example;

Let us take a normal pack of playing cards (52 cards - 13 Hearts, Diamonds, Spades, and Clubs) and I shall place the Ace of Hearts face down on the top of the pack.
Thus I am both necessarily connected to the Ace of Hearts (as I am to all matter in the universe) and I have pre-determined knowledge of the exact card, thus I can be certain that if I turn the card over it will be the Ace of Hearts. So we see that while I have complete knowledge of the system then there is no chance involved - the system is both necessarily connected and pre-determined.

Now let us further imagine that I place this pack of cards in front of someone else who has no knowledge of the fact that the Ace of Hearts is the top card. So while they are still necessarily connected to the top card (Ace of Hearts) they do not have pre-determined knowledge of this card, thus if I ask them to tell me which card is on top of the pack they only have a one in fifty two chance of guessing correctly. Thus we see how chance exists when we do not have pre-determined knowledge (even though we are still necessarily connected).

Further, in a necessarily connected but non determined universe, there are many possible futures (within the constraints of the properties of Space and the waves structure of matter - we can't flap our arms and fly!).

If we now apply this knowledge to the matter of brain and body (which is the cause of our human mind) we realise that while we are necessarily connected to the other matter in the universe, we can never have pre-determined knowledge of the motions of this other matter (as explained above due to the system being infinite) thus explaining how chance (due to lack of pre-determined knowledge) exists in the universe. Hence evolution of our brain and mind can use this chance to allow us to creatively think of new and novel ideas and relationships that we can then remember, and then we can use chance again to select from these various chance relationships to determine actions. Now while this is a very simplified explanation of how our mind works, and there may be millions of ‘layers’ to these chance selections, each with various degrees of probabilities for selecting different outcomes, nonetheless the principles are true and do explain how lack of pre-determined knowledge in a necessarily connected Universe within an Infinite Space allows chance and limited ‘free will’.

A simple example of this process can be found when considering a game commonly found in fairgrounds, where there is a clown's head with open mouth that is rotating backwards and forwards, and you must drop a ball and try and get it to land in a certain slot. Depending on the time that you drop the ball ultimately determines where it will land. But all options are possible, and if we use this chance (due to lack of knowledge, as explained above) to determine when the ball is dropped, then we realise that we are using chance to decide on the future of the universe.

Thus Spinoza is both right and wrong when he writes;

There is no mind absolute or free will, but the mind is determined for willing this or that by a cause which is determined in its turn by another cause, and this one again by another, and so on to infinity. (Spinoza, 1673)

His error was to not understand how lack of pre-determined knowledge in an infinite though necessarily connected system causes chance and limited free will.

Karl Popper also intuitively understood that there had to be some 'balance' between complete determinism (clocks) and complete disorder and chance (clouds) when he wrote;

What we need for understanding rational human behaviour - and indeed, animal behaviour - is something intermediate in character between perfect chance and perfect determinism - something intermediate between perfect clouds and perfects clocks. (Popper, 1975)

He was absolutely right, for a 'Necessarily Connected', but infinite and ‘Non-Determined’ space allows us to have limited free will (based upon this lack of pre-determined knowledge which gives rise to chance).

http://www.spaceandmotion.com/cosmos-determinism-free-will-necessary-connection.htm