It is easy to create an interstellar radio message which can be recognized as emanating unambiguously from intelligent beings. A modulated signal ('beep,' 'beep-beep,'...) comprising the numbers 1, 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, for example, consists exclusively of the first 12 prime numbers—that is, numbers that can be divided only by 1, or by themselves. A signal of this kind, based on a simple mathematical concept, could only have a biological origin.... But by far the most promising method is to send pictures.

Smithsonian magazine, May 1978, pp. 43, 44. Quoted in Awake! magazine, 1978, 8/22.