Originally Posted by Twistys Tim

If you look at things from SkyNet's perspective -- you would see that according to all calculations traveling through time to kill Sarah Connor was a fruitless endeavor, as John's existence in Skynet's present (in 2029) was proof that Skynet was unable to affect the present by altering the past.

If indeed Skynet could have effected change, then the present it was operating in would not have contained John Connor. 45 years earlier the machine it sent back to kill his mother was unsuccessful, as John's presence in 2029 attest to. Upon inventing the the time machine -- and being defeated, Skynet would have calculated that whatever actions it may undertake in the present / past will not result in it being successful.

Now we can look at Reese and John. In John's present prior to Reese being sent back to 1984, we can say that Reese has already traveled back in time, as without that event having already occurred John could not exist. But, John could have said to Reese "there is no point in traveling back in time to stop the machine -- as I am alive in the present, and therefore the machine will fail in it's attempts to kill my mother." If this were the case, and logically this is correct (John does indeed exist in 2029) and Reese did not travel back in time how would John's existence be explained?