- LOGIC
What differentiate man from other animals is the advanced thinking faculty he possesses. When religion and conscience fail as standards upon which the rightness or wrongness of a deed can be ascertain, man often times turns to logical reasoning. Logic promised a scientific certitude to moral precepts; it left the intellect free to inquire and to challenge; it threw man back on grounds which were found in this world alone, and could be tested by reason and experience; it derived no authority from antiquity, no sanction from religion; it stood entirely on its own feet, independently of the many conflicting elements which were found in the religions of the past and present. 
When faced with a dilemma to choose between two opposing deeds, one can use logic to reason out the consequences of the two deeds, weighing them against each other, determining the one with the greater good.
Basing morality on logic could be tricky because to apply, one must have abundance of facts to glee from. .jpeg)
So, one needs to rely on history of similar things. One failing thing about logic is that it depends mainly on one's way of thinking as well as been able to differentiate logical conclusion from sentiment which is of zero to low probability in most people.
To cap it, the best standard is evolution which encompasses religion, conscience and logic.
The moment morality is base on Evolution, one see that it must change with situations. There cannot be, it is obvious, one cut and dry rule for all.
The more one thinks upon and works out into detail this view of Morality as based on Evolution, the more we realise its soundness, and the more one finds that the moral law is as discoverable by observation, by reason, and by experiment, as any other law of Nature. If a man disregards it, either ignorantly or wilfully, he suffers.