gettys = ExampleData[{"Text", "GettysburgAddress"}]
Probability[
 x == {"t", "h", "e"} [Conditioned] y == {"t", "h"},
 {x [Distributed]
  EmpiricalDistribution[Partition[Characters@gettys, 3, 1]],
  y [Distributed] EmpiricalDistribution[Partition[Characters@gettys, 2, 1]]} ]