Deontology and utilitarianism are both abstract conceptions of ethics, and therefore, partial and defective. Their plausibility derives from two factors:
1) They each get at part of the truth: it is true, as deontologists insist, that principles are an important part of ethics. And it is true, as utilitarians contend, that the consequences of one’s actions are an important part of ethics.
2) Each approach is able to benefit from the defective nature of the other: so long as rationalism is understood as the only possible approach to ethics, then, to the rationalist, deontology appears to be the only alternative to utilitarianism, and vice-versa. So deontologists can strengthen their appeal by pointing out the obvious defects in utilitarianism (it ignores principles), while utilitarians do the same by noting the obvious defects in deontology (it ignores consequences). It is like a war between one’s right leg and left leg over which is the essential limb in walking: each leg can correctly note its importance to the activity, and also note the flaws in the argument of the other limb that it is exclusively essential to perambulation.