The original quote is mine. I say "by its very nature" since it is basically impossible to prove a negative (in this case "prove that there can be no spontaneous generation")
What is a contrapositive?
The contrapositive of a conditional statement is formed by negating both the hypothesis and the conclusion, and then interchanging the resulting negations.
In other words, the contrapositive negates and switches the parts of the sentence. It does BOTH the jobs of the INVERSE and the CONVERSE.
Example:
Conditional: "If 9 is an odd number, then 9 is divisible by 2."
Contrapositive: "If 9 is not divisible by 2, then 9 is not an odd number."