Suppose there is a given statement P(n) involving the natural number n such that 

(i) The statement is true for n = 1, 
  i.e., P(1) is true, and 

(ii) If the statement is true for n = k (where k is some positive integer), 
  then the statement is also true for n = k + 1, 
  i.e., truth of P(k) implies the truth of P (k + 1). 
  Then, P(n) is true for all natural numbers n.