Because all beings are composites of form and matter, the form of living beings is that which endows them with what is specific to living beings, e.g. the ability to initiate movement (or in the case of plants, growth and chemical transformations, which Aristotle considers types of movement).
bɪkˈʌz ˈɔːl bˈiːɪŋz ɑːɹ kˈɑːmpəzˌɪts ʌv fˈɔːɹm ænd mˈæɾɚ, ðə fˈɔːɹm ʌv lˈɪvɪŋ bˈiːɪŋz ɪz ðæt wˌɪtʃ ɛndˈaʊz ðˌɛm wɪð wʌt ɪz spəsˈɪfɪk tə lˈɪvɪŋ bˈiːɪŋz, ˈiː.dʒˈiː. ðɪ ɐbˈɪləɾi tʊ ɪnˈɪʃɪˌeɪt mˈuːvmənt ɔːɹ ɪnðə kˈeɪs ʌv plˈænts, ɡɹˈoʊθ ænd kˈɛmɪkəl tɹænsfoːɹmˈeɪʃənz, wˌɪtʃ ˈæɹɪstˌɑːɾəl kənsˈɪdɚz tˈaɪps ʌv mˈuːvmənt.