named Muhammad (pbuh) used to exist. How do we know that what is common about him is not a myth or a legend?"
You see, there are ways to know or gain knowledge in philosophy. One is that everything we see is called evidence. As we see the sun give light. So we say, yes the sun ge is also another source, called Testimony. To read a book, to hear from someone, these are testimony. As in the US presidential election, you didn't see it in person, you saw it in the news, you heard it through a friend / relative there, you saw it on Facebook. And you believe. These are all testimonial beliefs.
There are a few things to check when it comes to form a testimonial belief. The source from which I am gaining knowledge is telling the truth and he has not made any mistake. History says, science says, most of the knowledge we have on every subject is testimonial belief. As the earth is round and revolves around the sun, it is a testimonial belief. It is a testimonial belief that Sirajuddaula used to exist. Thus most of our beliefs are testimonial.
Now let's see whether the Prophet (peace be upon him) used to exist or not. His biography, his words, his works have all been preserved through a beautiful preservation process. That process is a science. There are certain methods of determining purity. This sequence of hadith preservation is called isnad. The sequence of hadiths can be traced from the present time to the Prophet (peace be upon him). The complete sanad and rabi tradition are recorded by the muhaddithin Whoever is in the narration of a hadith, the whole chain is preserved perfectly. It is a science that has been around for thousands of years.
Now let's see how these testimonials are verified. That is more interesting. The one who has narrated the hadith has to go through many criteria to prove his purity. The narrator's name, father's name, occupation and full identity must be known. Can't be unrecognized. Detailed news about him was taken. Whether his memory is weak or not, whether he lied or not, what his character was like, whether he was pious or not, these criteria are met. Hadith is not accepted if it is not correct. If a hadith is narrated by many narrators, its purity, and if it is narrated by few narrators, its purity is not equal. Hadith has been preserved for thousands of years by maintaining innumerable other such criteria. Dr. to know the details. You can read the book Ahiyaus Sunan by Abdullah Jahangir Rahimahullah.
That is to say, the biography of the Prophet (peace be upon him) and his words and deeds have been preserved through a series of testimonies. The testimony of the existence of Sirajuddaula is more likely to be weaker than the event of the existence of the Prophet (s) 1400 years ago.
Dear brothers and sisters, don't be alarmed by anyone's words. Study. If you don't know, you will accept the attack, you will accept the question, your faith will be weak. That is why Allah is the first to say in the Qur'an, 'Read'. May Allah protect the faith of my brothers and sisters.