A simple example of the formal cause is the mental image or idea that allows an artist, architect, or engineer to create a drawing.
ɐ sˈɪmpəl ɛɡzˈæmpəl ʌvðə fˈɔːɹməl kˈɔːz ɪz ðə mˈɛntəl ˈɪmɪdʒ ɔːɹ aɪdˈiə ðæt ɐlˈaʊz ɐn ˈɑːɹɾɪst, ˈɑːɹkɪtˌɛkt, ɔːɹ ˌɛndʒɪnˈɪɹ tə kɹiːˈeɪt ɐ dɹˈɔːɪŋ.