According to Aristotle, the plot events in a tragedy are related according to the principles of probability and necessity. The events must be probable and necessary in order for the tragedy to be effective.
In a tragedy, the event that is most important is the turn or reversal of fortune. The tragic hero must have a tragic flaw that leads him or her down a path toward destruction. There must be an element of inevitability in this process, which can only be achieved if everything that happens makes sense within the context of the play.
The events that make up the plot must be probable in order for them to be believable. If they’re too far-fetched or unbelievable, then they will not have any emotional impact on the audience. For example, if someone sees a character who is supposed to be dead suddenly appear alive again, this would not be considered “probable” because it’s hard to believe that a person could come back from the dead so easily.
What’s more, the events must also be necessary in order for them to make sense when put together into one story. This means that all of these events have some sort of connection with each other; they are related somehow. For example, if someone saw another person get killed by his own brother, then this would not make sense as part of one story because it would just seem random and unrelated to anything else happening in the play.