Elaine asked:
So… I’m a regular high school student who has run across the problem of not knowing what a monad is try so hard to understand what Leibniz what trying to explain but it seems like monads are just a substance of everything. They’re described as if they are alive yet they don’t exist and at the same time they already know what they’re suppose to do yet still nonexistent…*sigh* I’m trying to understand it all but in a way I can’t please help!
Answer by Martin Jenkins
‘Monads’ or ‘Monadology’ is Leibniz’s response to the philosophical debate started by Descartes. As Descartes had established the existence of two substances Extension (my body, your body, bodies generally, objects in the world, the world itself) and Mind (Thinking, unextended), one immediate problem arose as to how the two could interact as Descartes had argued they did.
Descartes never arrived at a satisfactory solution to this problem and it served to occupy thinkers such as Spinoza and Leibniz in their ponderings on the nature of Substance
The Problem of Substance
Following the influence of Aristotle, the nature of reality, of what is (Ontos Greek) was deemed to be that of Substance – the building block(s) of reality. Substances could be defined by their characteristics or Attributes. So the attribute of Mind was that it Thinks. Attribute of Body is that it is extended – has length, breadth, heights, can be measured and it occupies space.
If there are a plurality of Substances and not just one, then they will interact with each other – as Descartes had proposed Mind and Body do. This led to problems as to how to define, categorise and understand them. For if substance A causally interacts with substance B, – causes changes or effects in it – B will be affected by A and vice versa. Hence the clear water of substance definition becomes very muddied and it is hard to understand what part of a substance comes from another to the point where a pure substance and a corresponding definition of it becomes impossible.
The solution Spinoza proffered for this problem was that it was logically impossible for there to be more than one substance. Substance was self-generating and sustaining, it did not interact with any other substance and did not owe its existence to any other. In short, a single substance ruled out and problems associated with the causal relation of one substance to another. Spinoza deemed this single, all inclusive substance to be God or Nature (Deus sive Natura). Mountains, human beings, the computer screen before me are modifications of this single substance.
Why the detour with Spinoza? Because it contrasts and therefore provides an insight into the solution to substance interaction by Leibniz.
Leibniz
Each spirit [human being] is a substance. Leibniz calls them Monads. Like each human being, each monad has its own perspectives at any given time just as a building will appear differently from different perspectives. As they are created by God. God knows everything the monad will or will not do a-priori as God possesses true knowledge. In other words, God eternally knows the ‘what-ness’ and ‘is-ness’ (Haicceity) of each monad. This is stated in the example of Alexander the Great (#8 Discourse on Metaphysics. DM). That is, each Monad has an ‘individual notion’, its Idea which contains all it is, ever will be, ever will do.
As God knows the individual notion or idea of each Monad will do, the Monads cannot deviate from this as God knowing would be wrong – which as God is perfection, is impossible. So each monad is, so to speak, like clock-work: it is wound up by God and in unwinding, it lives its life: everything the monad does in its life is contained in and determined by the ‘winding-up’. (#13. DM)
As so ‘wound-up’ by God, each Monad will do its own thing, regardless of any other Monad. This is what is meant by the term ‘windowless monad’, there is no influence from outside the monad, it is enclosed in itself and executes its own life as determined by God’s knowledge. Hence as Leibniz writes that each monad expresses the universe in its own way. (#9. DM)
So the problem of substance interaction and knowing the nature/identity of each substance or monad is solved by Leibniz.
There is no influence from any other monad into/onto another. It may appear so as when two people bump into each other for example, but again; God has already anticipated this as he knows it by knowing our individual notion. As he knows it, the interaction of one monad with another is pre-established by God and everything occurs according to this divine knowing and not by our individual will. Hence, it is all a matter of ‘pre-established harmony’. (#15. DM)
Monads do exist! They are created by God and depend upon it for their existence (emanation). They can also be called ‘souls’. Only God can create and end the existence of a Monad. As there are millions of monads, there are millions of corresponding views or perspectives of the Universe by each monads. This is in actuality, the view each of us has from the standpoint of our own, individual lives.
Hope this is of use regular high school student Elaine.
Ask a Philosopher 
Fascinating!