The Third Foundation is the realization that we exist. This is implicit in the Second Foundation, that the world exists and that we are a part of the world. However, a rock is also a part of the world, and yet it cannot know this. We are able to know that we are a part of the world. We are able to be aware of not only the world itself, but of our selves. We are self aware. More important than the world existing, WE exist!
I AM! Revel in it. It is the only thing you can be absolutely sure of. True, your senses can be decieved. So again, we must choose to make this assumption. Either we exist, or we don't. If we choose that we do not exist, none of this matters. We can no longer make choices, or observations. We can not continue any further. Thus, this choice is not acceptable. We must choose that we exist. We must acknowlege this. We have no choice, really. If we say no to this, we fall into the void. So, we are.
Still, nothing is true. That we exist is an assumption. However, it is one which we cannot argue without ceasing to exist. Thus, this assumption becomes a foundation. Also, just being a part of the world is not enough. We can observe that we can observe ourselves. We have self awareness. Self awareness is consciousness. We are conscious. This we know. This assumption we must accept as true.
This is also untimately the only thing we can really say about ourselves. Sure, we are possessed of great knowlege and skills. But remember, none of these can be known as true. All that we know is that we are. This is the one description we can be sure of about ourselves: we are.
"I am". That is the answer God gave Moses when he asked him who is was. God spoke the only true answer he could give. The same is true for us. That story is an example we can apply to ourselves. It does not matter if you believe the events of the story, the principle demonstrated is sound. All I can say that is true in describing myself is that "I am". For this is the only thing I know.
Even if I forget all other things. Even if I know nothing, and all that I have falls into the void. Even if the world itself were to disappear. I will still know that I am, for as long as I am able to concieve this idea, it is true.
I am. This is the Third Foundation.