# Introduction to Philosophy/Logic/Paradoxes

Introduction to Philosophy > Logic > Paradoxes
    The simplest way to think about the paradox is as something that can not exist without itself. Generally, it is an event perpetrated in such a way as to remove or render impossible the perpetration of said event. There are many forms of paradoxes. Three such are the predestination paradox, the ontological paradox, and the grandfather paradox.

## Several Types of Paradoxes
### Predestination Paradox (Causal Loops)

A predestination paradox, or causal loop, is a paradox of time travel. It exists when a time traveller is caught in a loop of events that "predestines" him to travel back in time. This paradox is in some ways the opposite of the grandfather paradox.
The following are two examples of a predestination paradox:
   As an example, say there is James, a time travelling android. He was created by Mark, who found the plans to make James on his dresser one day and worked from those. The issue at hand is that the plans were written and placed there by James after he was made. The obvious difficulty is the fact that, at some point, the cycle has to have an outside factor as a beginning.
   Say that Irwin was born as a result of Nick dying in an assassination. Irwin has always believed that Nick's death was a terrible loss to humanity, and he goes back in time to prevent it. In a universe of diverging realities, this may make sense, as Irwin is actually travelling from reality A to reality B, where he can prevent the assassination and maintain his existence. Likewise, there may be a universal principle that prevents such paradoxes from occurring that holds him back from doing anything, however, in the constraints and for the purposes of this discussion, we will assume there is no such principle. Irwin warns Nick (or pushes him down, or shoots the sniper, or removes Nick from the situation), thereby erasing his birth from existence. The question raised is, how was Irwin born to go back in time in the first place, as he was not born at all supposing that the cause of his birth (Nick's death) has been removed? As such, he couldn't have protected Nick from assassination, else he would not be born. This, in turn, would lead to him being born, which would mean that he went back in time and saved Nick, and the same process would be repeated ad nauseam.

### Ontological Paradox

An ontological paradox is similar to a predestination paradox in that it is nested in causation, but raises the ontological questions of where, when and by whom the schematics were created. Time loop logic operates on similar principles, sending the solutions to computation problems back in time to be checked for correctness without ever being computed "originally."
   One example is seen in Star Trek II: The Wrath of Khan and Star Trek IV: The Voyage Home. In Star Trek II, Captain Kirk receives antique eyeglasses from Dr. McCoy. In Star Trek IV, Captain Kirk pawns the glasses (nearly three hundred years in the past, in the late 20th century) given to him by Dr. McCoy, who will, again, buy the glasses for Kirk in the 23rd century.

### Grandfather Paradox

The grandfather paradox is a paradox of time travel, supposedly first conceived by the science fiction writer René Barjavel in his book "Future times three" ("Le voyageur imprudent", 1943). Suppose you travelled back in time and killed your biological grandfather before he met your grandmother. Then you would never have been conceived, so you could not have travelled back in time after all. Now did you travel back or not? The grandfather paradox has been used to argue that backwards time travel must be impossible. However, other resolutions have also been advanced.
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