As J.S. Mill noted in his work On Liberty, “the practical principle guiding opinions on conduct is each person’s feeling that all should be required to act as he would like.” Because of this, as Mill notes, “Men very rarely chose a side because of a consistently held opinion about what is fit to be done by government.” This tendency extends across the political spectrum and is not an ill specific to any ideology.
As an example of inconsistent application of principles, consider the principle of local rule, which is lauded by conservatives when the locals are doing what they want them to do and rejected when they are not. To illustrate, consider the matter of fracking. When local governments move to ban fracking in Republican controlled states, the same Republicans who praise the principle of local rule quickly violate that principle in favor of the oil companies who want to frack.
While liberals do not use the talking point of local rule as much as conservatives, they are also inconsistent in their view of the matter. When the federal government is doing what they like (such as legalizing same-sex marriage) they are fine with local rule being overruled. When the state or federal government is not doing what they want (such as cracking down on sanctuary cities or limiting abortion access), then they favor local rule that matches their views.
As another example of inconsistent application of principles, consider the matter of executive orders. When Bush was issuing executive orders, conservatives argued in favor of the President’s right to do so while liberals often argued that this was an overreach of presidential power. When Obama took office, their positions reversed on this matter. This showed that there was not a principled view of executive orders, just that people think it is okay when their guy is doing what they want and not okay when the other guy is doing what they do not like.
A principle is consistently applied when it is applied in the same way in relevantly similar circumstances. For example, if a person consistently held to a principle of local rule, then they would apply that principle whether they liked what the locals were doing or not. So, if the locals decided against fracking or in favor of allowing bakers to refuse to sell wedding cakes to same-sex couples, then they would apply the principle of local rule to these equally.
At this point some readers might be worried that following the principle of consistent application could lock people into supporting terrible things. For example, if a person had a principle that supported local rule, might they thus be obligated to support the locals if they voted to legalize heroin or cannibalism? Fortunately, the principle of consistent application is not an absolute principle and can be overridden by other moral principles, such as principles against cannibalism. Another key consideration is the principle of relevant difference. In general terms, a principle can only be applied differently in situation A and situation B if there is a difference between A and B that justifies the difference. For example, a person may favor local rule regarding fracking bans while opposing a local rule allowing cannibalism based on a relevant difference between the two. In this case, they still hold to a principle of local rule consistently—it is simply overridden by the relevant differences between cannibalism and fracking bans.
One rather obvious problem is sorting out what counts as a relevant difference that justifies applying a principle differently. As would be imagined, people will tend to find what they think are relevant differences whenever they wish to not apply their principle in the same way. For example, a conservative might claim that while local rule is fine when a locality wants to allow local bakers to not sell wedding cakes to same-sex couples they might think there is a relevant difference between that and allowing locals to ban fracking. As another example, a liberal might favor restrictions on hunting because they oppose killing living things, while claiming that there is a relevant difference between these restrictions and restrictions on abortions.
One limiting factor on relevant difference is consistency: if a relevant difference is claimed between two things to justify a difference application of a principle, then that same difference would apply in all relevantly similar cases. Naturally, people can engage in ever finer distinctions in the differences in efforts to justify applying their principle in different ways, thus making the battle over relevant difference quite contentious. However, consistent application is still important—which leads to the subject of why it matters.
One reason why consistent application matters is that it is critical to being an ethical and principled person. After all, a person whose “principles” are too flexible does not really have principles—they simply do what they want. This can, of course, be countered by arguments against being principled and in favor of simply doing what one wants. One example of this approach in ethical theory is ethical egoism.
Another reason why consistent application matters is logical—if a person uses a principle to justify something and also acts in a way that violates the principle, then they have engaged in inconsistency. This means that they cannot be right in both cases—they must be wrong at least once. For example, if a person justifies state laws that allow greater pollution than federal laws allow by appealing to states’ rights and then opposes state laws that are more restrictive than federal laws by arguing that the federal government should set the regulations, then at least one of their arguments must be flawed. This is because if the principle of states’ rights justifies states in having weaker regulations than the federal government sets, then states’ rights would also justify states in having stronger regulations than the federal government sets. Naturally, an appeal can be made to the principle of relevant difference—the challenge of doing so will vary. In the states’ right example, it would clearly be challenging to argue that states’ rights only apply to states that want weaker standards and not to those that want stronger standards.
From a logical and moral standpoint, showing that a principle is being applied inconsistently is a strong criticism—it shows that those applying the principle are wrong in at least one case. From the standpoint of persuasion, it tends to be far less effective—people who like the inconsistent application will probably not be swayed by the charge of inconsistency and those who dislike it will tend to not need and additional reason. That said, one interesting tactic might be to use a recording of the person being inconsistent and use their own word to argue against them. Then again, while this can be amusing, it would probably still lack persuasive power since people want what they want, consistency be damned.