Sentimentality originally indicated the reliance on feelings as a guide to truth, but current usage defines it as an appeal to shallow, uncomplicated emotions at the expense of reason
I cannot understand this sentence. What's the meaning of "appeal" in this sentence? And "at the expense of reason"?
...an appeal to shallow, uncomplicated emotions at the expense of reason.
...an attempt to persuade by arousing emotions, sacrificing or contradicting logic.
OP's problem with appeal may be because the original meaning was strongly associated with legal/rational argument. It still has that sense, in that you can appeal against a court judgement, for example. But nowadays it's so often used in the "attraction" sense that we even use the acronym SA for "sex appeal".
The usage at the expense of (sense 7) is a set phrase used where having more of one thing (emotional appeal, here) implies less of another (rational and coherent argument).
To make it easier for you to understand, the sentence may be rewritten/reworded to:
Sentimentality originally indicated the reliance on feelings as a guide to truth, but current usage defines it as an appeal to shallow emotions at the expense of reason.
To make it even more easier:
Current usage defines sentimentality as an appeal to shallow emotions at the expense of reason.
"At the expense of reason" means to do something not caring about reason, not being rational. So in the end, the sentence says that when you're sentimental, you tend to have shallow emotions and you don't think rationally; you don't put reason to your thinking.
There are two lookups you should make
1) idiom at the expense of
Fig. to the detriment of someone or something; to the harm of someone or something. He had a good laugh at the expense of his brother. He took a job in a better place at the expense of a larger income.
This figurative meaning developed from the expense as cost, meaning that when you say at the expense of something it implies that something will in some way pay for it.
2) dictionary entry for appeal, 2.b list an example very close to yours
an attempt to make someone do or accept something as right or proper by saying things that are directed at a person's feelings, attitudes, etc. — + to ▪ The author makes an appeal to the reader's emotions. ▪ an appeal to reason ▪ an appeal to the intellect
This is not much of a definition, but more a description, so in case you have problems with it, look at synonyms of this sense (request for help): address, invocation, imploration, solicitation, plea.
So substituting the original sentence:
Sentimentality originally indicated the reliance on feelings as a guide to truth, but current usage defines it as an appeal to shallow, uncomplicated emotions at the expense of reason.
to
Sentimentality originally indicated the reliance on feelings as a guide to truth, but current usage defines it as a plea to shallow, uncomplicated emotions that hurts the logic.
This substitution is not perfect - it makes the statement more accusing (hurt is stronger than expense on average) and it was done more to show how the structure works.
One note - the phrase 'shallow, uncomplicated emotion' is not sound; sentimentality can appeal to deep emotions.
Appeal here means “a request or reference to some person or authority for a decision, corroboration, judgment, etc.” (From dictionary.com, sense 2.)
At the expense of here means “to the detriment of”. (From dictionary.com, sense 7.)
So the claim in the sentence is:
Sentimentality now means a type of persuasion which calls upon shallow, uncomplicated emotions that interfere with the use of reason.
I use 'appeal' to mean a request. However, it's not just any request, it's a request for someone to change their opinion or stance on something, often right before or after a major decision. It can often involves begging or underhanded behavior, but doesn't have to.
You would appeal a verdict in a court, you might appeal being sent on time-out, you might even appeal to a bully's better nature in trying to get them to back down.
You would probably not appeal someone's decision to go to the movies, or appeal someone's decision on which fruit and vegetables to buy.
In this circumstance, the sentence claims that sentimentality describes a wheedling, begging, underhanded attempt to 'win' an argument, debate, or other disagreement by evoking an emotional response, instead of reasoning out the right answer.
Let us imagine an illustrative scenario in which two people are arguing over, in a loss of airplane cabin pressure, whose oxygen mask one should put on first. Person A says that you should put on your own mask first, because then you're more able to help those around you. Person B says "HOW CAN YOU BE SO SELFISH! THINK OF THE CHILDREN! THEY GET A MASK FIRST! WHAT HEARTLESS MOSNTER WOULD PUT ON THEIR OWN MASK FIRST?!"
Person B's argument would be an example of sentimentality, as described in the quoted line provided by the OP. Obviously, their reasoning, if they had any, is wrong. Instead, they're trying to trigger an emotional response, to appeal to emotions, to win the point.
