Is finding a teacher necessary to practise dhamma?

I never had a meditation teacher in real life but I listened their dhamma talks etc.. through using modern day's technology. I think teacher has a very important role for a meditator's progress in his/her path of freedom from suffering. Because teacher tells all of the details in meditation practise that is necessary for the meditator to know, the teacher guides and helps the meditator in the difficult periods of the meditator's spiritual path. Also the dhamma talks that the teacher gives is more than just informations and teachings about Buddhism, meditation, etc.. but it has the effect of changing the mind of the meditator. The dhamma talks literally have the effect of purifying the meditators mind. That's why many people in Buddha's time became enlightened just by listening to dhamma talks. But in most cases, it is necessary to practise meditation first to get that benefit from the dhamma talks.

So as a person who never had a meditation teacher in real life(but followed them through using internet), I think that it is possible to keep your practise alone, but it is better to have a teacher in real life. If you don't have a meditation teacher in real life you can follow meditation teachers by using modern day's technology. That's also kind of a teacher-disciple relationship, but it is not good as having a teacher in real life.

Even though the monks, full-time practitioner, still required the Buddhist Meditation Professors, why do lay people think they can understand Tipitaka by themselves and enlighten without teacher's taking care? Are they already have memorized Tipitaka enough to taking care of themselves?

The knowledge, experience, and proficiency are very important.

People cannot manage their listened to others' perspectives, so they hate to listen to the others' perspectives.

People will know how to understand the others' perspectives by the Buddhist Meditation Professors' advis*m*nt, then they can be happy to listen to the others' perspectives.

"People's heart is a deep ocean of secrets." The methods to heal and meditate people's heart are so complicated as well, so Tipitaka Memorizer is required to teach those complicated methods like a doctor who took care of his patient.

The similitude is the chemist who has recited, memorized, and learned about the periodic table and the formulas from the past chemist generation to generation.

If the science students want a science teacher, the professor is required, not just want, for Buddhist students.

In Vinaya-Pitaka Mahakhandhaka, Bhikkhu can't live alone without his teacher. Buddha blames and makes a law that a bhikkhu must lives with a Tipitaka Memorizer every day until he can finish Nissayamuccaka Course. Even though he already finished the course, he still has to live with Tipitaka Memorizer until his ordination time is five years old.

It's legal punishment as "dukaṭa-āpatti" for the new Bhikkhu who sleep without his professor in the same monastery every morning, each new offense per day.

Even bh.Potthila, Tipitaka Memorizer, he still needs Tipitaka Memorizer Ariya to teach him the method to enlighten.

In the path of purification and Atthakathā said it required Ariya teacher to teach the meditation and go to meet him about one time per week. And it is better if that Ariya is Tipitaka Memorizer because Tipitaka Memorizer Ariya has more tips and tricks from Buddha than normal Ariya.

The path of purification Part II—Concentration (Samádhi) CH. III TAKING A MEDITATION SUBJECT page 94 ^(online):

1. Because of the words beginning, “Ánanda, it is owing to my being a good friend to them that living beings subject to birth are freed from birth” (S I 88), it is only the Fully Enlightened One who possesses all the aspects of the good friend. Since that is so, while he is available only a meditation subject taken in the Blessed One’s presence is well taken.

2. But after Buddha final attainment of Nibbána, it is proper to take it from anyone of the eighty great disciples still living.

3. When they are no more available, one who wants to take a particular meditation subject should take it from someone with cankers destroyed, who has, by means of that particular meditation subject, produced the fourfold and fivefold jhána, and has reached the destruction of cankers by augmenting insight that had that jhána as its proximate cause. But how then, does someone with cankers destroyed declare himself thus: “I am one whose cankers are destroyed?” Why not? He declares himself when he knows that his instructions will be carried out. Did not the Elder Assagutta [99] spread out his leather mat in the air and sitting cross-legged on it explain a meditation subject to a bhikkhu who was starting his meditation subject, because he knew that that bhikkhu was one who would carry out his instructions for the meditation subject?

4. So if someone with cankers destroyed is available, that is good. If not, then one should take it from a non-returner, a once-returner, a stream-enterer,

5. an ordinary man who has obtained jhána,

6. one who knows three Piþakas, one who knows two Piþakas, one who knows one Piþaka, in descending order [according as available].

7. If not even one who knows one Piþaka is available, then it should be taken from one who is familiar with one Collection together with its commentary and one who is himself conscientious. For a teacher such as this, who knows the texts, guards the heritage, and protects the tradition, will follow the teachers’ opinion rather than his own. Hence the Ancient Elders said three times, “One who is conscientious will guard it.”

   Now, those beginning with one whose cankers are destroyed, mentioned above, will describe only the path they have themselves reached. But with a learned man (Tipitaka Memorizer), his instructions and his answers to questions are purified by his having approached such and such teachers, and so he will explain a meditation subject showing a broad track, like a big elephant going through a stretch of jungle, and he will select suttas and reasons from here and there, adding [explanations of] what is suitable and unsuitable. So a meditation subject should be taken by approaching the good friend such as this, the giver of a meditation subject, and by doing all the duties to him.

   If he is available in the same monastery, it is good. If not, one should go to where he lives.

Do you need teacher to practice Buddhism?

No you don't - but you need to know how to practice. And with so much contradicting information about Buddhism on the Internet and in books, to know the right practice requires insight into principles of Buddhism. Once you understand the principles, you can see for yourself which practice is real and which is a confused echo of real.

Do you need teacher to gain insight?

No, you don't. You can piece the puzzle together from books and videos, and integrate it all together into your own insight.

However, that requires an extremely good memory, very good intuition for the implied/inferred meaning, and a very good ability to connect the pieces of information that you may have received in different context, many years apart.

In real life, I'm a professional software architect at a large US company. As a software architect, I have an extremely well-developed capacity for remembering thousands of little pieces of information that I don't fully understand, and then slowly integrating it together into a meaningful whole that I can reason about and often see problems that others can't see.

And even with this ability, it took me almost 15 years of studying Buddhist texts, across Theravada, Mahayana, Vajrayana, Zen - to have even the basic understanding of how it all fits together. I'm not talking about the theory, I'm talking about understanding the basic idea, the framework, the "spirit" of Buddhism. Knowing the theory without understanding the "spirit" is useless. Someone like that can quote the texts but can't explain it in his own words. It's only when you get the "spirit" is when you "find the ford" and become "independent of others in regards to the Buddha's message" (the label on my profile).

Anyway, imagine my surprise, when after those 15 years I went to a live Zen Master and from his few replies and the overall behavior realized that he had a perfect intuitive grasp of all the principles that it took me 15 years to even begin to discover! I could have saved so much time! More than that, in two years of practicing meditation under Master Shin, I learned more about the real meaning of Buddhism than in 15 years of reading books.

Alternatively, you could in theory repeat Buddha's path and figure it all out in meditation, without any guidance or books, - but unless you're a genius like the Buddha (although as @PeterJ pointed out, even the Buddha had some teachers), the chance you'll strike gold is one in a billion.

So, do you need teacher to gain insight? No. In fact, it is impossible for any teacher to give their insight to someone. Insight is something that happens in your head, as you build the puzzle and see what it means in real life. But a live teacher who already has the insight, can save you years or even decades of time you'd either spend accumulating theory (like I did) or stumbling in the dark.

So even though I myself spent 15 years studying books and only 2 years learning from teacher, and now 6 years practicing on my own again, when anyone asks this question I always say: yes, go to a live teacher. If you can't find a live teacher, then at least talk to me. I will do my best to share as much insight as I can. Do not repeat my path.