Plainly put, the formal cause is the idea in the mind of the sculptor that brings the sculpture into being.
plˈeɪnli pˈʊt, ðə fˈɔːɹməl kˈɔːz ɪz ðɪ aɪdˈiə ɪnðə mˈaɪnd ʌvðə skˈʌlptɚ ðæt bɹˈɪŋz ðə skˈʌlptʃɚɹ ˌɪntʊ bˈiːɪŋ.