Aristotle proposes in his

Poetics

that the unity of tragic plot must be achieved by adhering to the principle of “probability or necessity”. However， the application of these modal terms to constrain the composition of tragic poets raises numerous questions. By examining the philosophical implications of these modal terms， it becomes evident that “probability” is more suitable than “necessity” for describing the human actions imitated by tragedy. Furthermore， according to Aristotle’s recapitulation of the plot of

Iphigenia in Tauris

， the principle of probability requires， on the one ha

nd， that the tragic plot present the causal connections among events in human life， thereby highlighting the philosophical value of poetry； on the other hand， that the choice of action based on character reveals the inherent moral-philosophical dimensions of the tragic plot.