Please explain what counts as non-arbitrary in the context of motivations and moral imperatives (e.g. thou shalt not kill, create the greatest good for the greatest number, etc.).
Dictionary definitions of "arbitrary" are: "based on random choice or personal whim, rather than any reason or system", "depending on individual discretion (as of a judge) and not fixed by law",...
While comprehensible, these seem feeble and vague in the case of moral imperatives. For example, the principle "thou shalt not kill" can be explained by evolutionary processes and its effect on human psychology that causes aversion to killing. Or by sociological strategies: a community that kills its own members tend to be weaker and have less solidarity than those that do not. These explanations state the physical/psychological justification of the moral principles, but can nevertheless be argued to be as arbitrary as the processes/strategies explaining them.
As for motivation, biological justification for the motivation/compulsion/desire for things like sex and sugar consumption is mountainous, yet one may say that these desires are only meaningful for organisms with internal fertilization and the ability to digest sugar, and thus arbitrary.