Arguments to do with why we should be moral

Aristotle

How can we work out what qualities etc. we need to be succesful human beings? Aristotle proposed the following:

We work out whether a thing is good or not by referring to its function. 

Example: A knife's function is to cut things. So a good knife needs to be strong, sharp, and quite light. A knife with those qualities is good. 

Aristotle said we can apply this to humans. By examining their function, we can find out whether or not they are good human beings. 

The function of a human being is the activity of the part of the soul with reason. Therefore, a human being is good if they exercise the part of the soul with reason well (i.e. with virtue) 

This is the view that each person has one ultimate aim: his own welfare.

Why should we believe this? 
Informal evidence: that's how people seem to behave. E.g. cutting in queues.
Also, this is how we're portrayed in terms of law. The sanctions of law don't appeal to our better nature, but to our self-interest. E.g. if you kill or steal, you will go to prison.

Objections: 
Non-self-regarding desires: a person sometimes acts on desires that are not self-regarding. E.g. a soldier throwing himself on a grenade to protect his team from the blast.
Reply: The psychological egoist can explain these in terms of a desire to be remembered as a hero or to avoid the guilt of letting other people suffer.
Empirical evidence: there is evidence that people act altruistically. E.g. people continue to help others even if it isn't convenient for them to do so. 

This is the view that everyone should maximise their self-interest. Even if it's not true that everyone always acts out of  self-interest, we can consistently claim that everyone is morally obliged to do so. 

Why should we believe this? 
It fits with what we think about human motivation.
It generates moral judgements we may find intuitively acceptable (e.g. stealing or lying).

Weaknesses:
Divergences between what an ethical egoist should do and what we think intuitively is morally acceptable. E.g. an ethical egoist might spend their money on an original work of art, but some people would argue that giving money to charity, if you have it, is obligatory.
Reply: an enlightened egoist may recognise the benefit to himself of helping correct systematic inequalities.  

'A good man cannot be harmed in either life or in death' (Socrates in Apology 41c)

Virtue theorists maintain that it's in your interest to be moral: they argue that possessing and exercising the virtues is constitutive of a eudaimon life. This means virtue ethics is formally egoistic. 

This seems to diminish the stature of virtue ethics as we appear to only be acting morally because it's in our self-interest which is distasteful surely? Aristotle disagrees - 
An egoist appears distasteful because they make themselves the prinicipal beneficiary of an act. They seek personal advantage through the disguise of being moral. Aristotle says the virtuous person, by contrast, performs the right action at the right time for the right reason. It is the consequence that gives them a worthwhile life, but not their main purpose for performing the act.

Virtues = excellent character traits. E.g. courage. The opposite of a virtue is called a vice. E.g. rashness. 

Eudaimon life = a worthwhile life. Aristotle thinks a truly worthwhile life is one of virtue and contemplation. 

This is the view that it's in our interest to be moral. We are better off being moral than not. 

This view puts humans as egoists. But how does acting morally benefit us? One way of explaining this is Hobbes and the state of nature: 

Hobbes claims the state of nature for human beings is conflict. We have competing desires and seemingly limitless appetites. Given an environment of scarce resources we will ultimately end up fighting. Hobbes says the state of nature would be a state of war. Hobbes assumes that a) human beings have ever increasing appetites, and b) there is a limited supply of food. 
The hideousness of this prospect motivates humans to make a contract whereby they transfer their rights of self-determination to the sovereign on the understanding that everyone else does this too.
This improves a person's situation and we can see why they would abide by such a contract. 

Weaknesses - Not clear whether this account of moral motivation would cover all cases. E.g. babies can't act reciprocally but we feel morally obliged to help them and not harm them.
This view can't account for the origin of morality: since a contract presupposes the existence of some moral values, we can't explain the originality of morals in terms of a contract. 

Kant's theory
Kant thinks that (i) a person's action has moral worth if he does the right thing for the right reason and (ii) a reason is moral if it can consistently be willed to a universal law. 

(i) A person must act because it's the right thing to do for the act to be moral. They cannot be motivated by pleasure, happiness or honour. Another way of putting this is you must act for the sake of duty. 

(ii) Take the example of lying. Kant thinks liars are non-rational. This may seem quite odd but Kants believes that in order for it to be rational to act on a particular reason that reason must be capable of being a universal law. A universal law (or law that could govern everyone) is the categorical imperative. This is something you must do, regardless of the situation or any inclinations you have. Therefore, lying is never right in any situation. 

Weakness
-What do we do if we are morally obliged to do two conflicting maxims? I.e. we can't do one if we are going to to do other. It is our duty to do both but it's also impossible to do more than one. e.g. giving money to charity and feeding my family.