What is a word to describe one's thoughts that sound logical and reasonable but, in reality, they are not? [duplicate]

The word you may want to consider is sophist. What they do is called sophistry or sophism.

https://www.lexico.com/definition/sophist

1. A paid teacher of philosophy and rhetoric in Greece in the Classical and Hellenistic periods, associated in popular thought with moral scepticism and specious reasoning.

1.1A person who reasons with clever but false arguments.

https://en.wikipedia.org/wiki/Sophist#Modern_usage

In modern usage, sophism, sophist and sophistry are used disparagingly. A sophism is a fallacious argument, especially one used deliberately to deceive. A sophist is a person who reasons with clever but fallacious and deceptive arguments.

https://www.lexico.com/definition/sophism

A clever but false argument, especially one used deliberately to deceive.

https://www.lexico.com/definition/sophistry

1. The use of clever but false arguments, especially with the intention of deceiving.

1.1 A fallacious argument.

If you are looking for the adjective, the word is sophistic, but I do not think it should be used for describing people, and it seems rare.

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On the other hand, we have casuist

https://www.lexico.com/definition/casuist

A person who uses clever but unsound reasoning, especially in relation to moral questions; a sophist.

How about specious?:

1 Superficially plausible, but actually wrong.

‘a specious argument’

More example sentences

1.1 Misleading in appearance, especially misleadingly attractive.

‘the music trade gives Golden Oldies a specious appearance of novelty’

If you are looking for a noun, "a fallacy" comes to mind. The adjective being "fallacious" ( a fallacious conclusion, a fallacious argument).

A fallacy is the use of invalid or otherwise faulty reasoning, or "wrong moves" in the construction of an argument. A fallacious argument may be deceptive by appearing to be better than it really is. From Wikipedia

fallacy - "an often plausible argument using false or invalid inference."
e.g.
"The fallacy of their ideas about medicine soon became apparent."
"The once-common fallacy that girls just weren't any good at math."

A fallacy is reasoning that comes to a conclusion without the evidence to support it. This may have to do with pure logic, with the assumptions that the argument is based on, or with the way words are used, especially if they don't keep exactly the same meaning throughout the argument. There are many classic fallacies that occur again and again through the centuries and everywhere in the world. From MW