More than 50% of crops and soy we produce is used to feed animals in factory farms. Meanwhile, there are millions of people starving to death around the world.

Animal agriculture is responsible for more than 50% of the deforestation.

Pigs are, by some measures, way smarter than dogs.

About Vegetarianism

Vegetarianism means abstaining from eating meat. It usually applies to the meat of the birds and mammals, the animals most neuroscientists agree are capable of feeling pain. So, why do I adhere to vegetarianism?

Valuing everyone's freedom,
regardless of how diverse
they are,
is a core of many peoples'
ideologies. However,
hardly anyone is consistent
enough to apply that to animals
of other species.
Well, when I started to explore the world using the Internet, one of the first things I found out was that the reasons usually given for eating meat are nothing but modern-day myths.
It's said that eating meat is a natural and healthy way to get the Vitamin B12. However, chances are, the meat you can buy in a supermarket doesn't even contain it. Vitamin B12 is produced by the bacteria in the intestines of some animals. Today, the farmed animals are being regularly fed with antibiotics, which, among other things, kill those bacteria. It's often said that meat is a good source of the Omega-3 fatty acids, which prevent heart disease. Firstly, there isn't much evidence that Omega-3s help prevent heart-disease, and, in fact, there are some good reasons to think they don't (UPDATE on 27/11/2020: Let's try to analyze this issue as scientifially as possible. There are, as you probably know, two basic types of cholesterol, high-density cholesterol (HDL) and low-density cholesterol (LDL). LDL causes heart disease, we know that for certain because there are many pills that cure heart disease by lowering the LDL. As for HDL, as far as I can tell, the best evidence we have suggests it has no effect whatsoever on heart health. However, if you ask me, there is a huge economic (and therefore political) incentive to make people believe HDL lowers the risk of heart disease. Omega-3-acids, found in flax, avocados and fish, to simplify, convert LDL to HDL. Low levels of alcohol raise HDL without significantly affecting LDL. Saturated fat in coconuts, milk, eggs, meat and avocados raise both the LDL and HDL. If HDL lowers the risk of heart disease, then flax is superfood, alcohol is not as harmful, meat and coconuts are not as harmful (or maybe not harmful at all), and so on. But there is no good evidence of any of that. Yet, you cannot deny there is a huge incentive to make people believe that.). Secondly, animals get the Omega-3 from grass, and most of the animals farmed for meat today never see the grass, they spend their entire lives in the factory farms (UPDATE on 27/11/2020: A common counter-argument I get when I state that are the statistics that only 14% or 18% of food given to cows worldwide is grain suitable for humans (implying the rest of it is grass). The first problem with that is that the rest of it is not actually grass, but mostly high-fiber corn grown specifically for cows because their digestive system needs a lot more fiber than our needs. High-fiber grain contains no omega-3 acids either. Second, those statistics are rather misleading because they ignore the vast difference in feed conversion ratio. Feed conversion ratio for cows fed with grain suitable for human consumption is around 2 to 1. Feed conversion ratio for cows fed with high-fiber grain is around 7 to 1. On the other hand, feed conversion ratio for cows fed with grass is so high it cannot even be reliably estimated, estimates range from around 20 to 1 to more than 50 to 1. So, if 10% of cows were fed only with grass, and 90% were fed only with grain suitable for humans (impossible, but I think this is good for illustrative purposes), those statistics would show 50% of food given to cows is grass. See how misleading those statistics are?). The third argument used to justify eating meat is that it's a way to get enough protein. But, here is a thing, we don't actually know how much protein an adult human being actually needs. The figure of 35% of the energy intake written in many biology textbooks is without a doubt a huge overestimation. There are some experts who think that the reason there is so much osteoporosis these days is, in fact, that peoples blood pH is too low because they are taking too much protein. I am not saying they are right, I am just saying there isn't much evidence for some of the well-known things about nutrition (UPDATE on 27/11/20120: As far as I understand it, most scientists agree the current epidemic of osteoporosis is caused simply by Vitamin K deficiency, which makes studying whether too much protein is to blame for osteoporosis difficult and pointless.).
Some of the other arguments used to justify eating meat is that animal agriculture supposedly helps the environment by fertilising the ground. The fact is, it does exactly the opposite. Actually, it hurts the environment at least as much as the modern traffic does (UPDATE on 27/11/2020: Many people will, in response to arguments about vegetarianism, say stuff like "Well, it does not need to be done that way. We can use animals to make use of the infertile land on which we cannot grow plants for human consumption. They may even make the ground more fertile.". This idea is known as regenerative agriculture. Well, there is a number of problems with that. First of all, pigs obviously cannot be used that way, since they have almost the same nutritional needs as humans have. So, it is still unethical to eat pigs, and probably chicken as well. Second, much of the land that is not suitable for crops also is not suitable for cows because it is too steep (though some more rarely eaten animals, such as goats, can be used that way). Third, land that is infertile for crops tends to be fertile not only for grass edible by cows, but also poisonous plants. That is less of a problem with modern veterinary medicine, but it still is a serious problem that makes regenerative agriculture unscalable. Fourth, however messy this part of ecology may be, all the scholars agree a massive switch from grain-fed cows to grass-fed cows would be a bad thing for the climate, because grass-fed cows emit much more methane. Fifth, the evidence we have that grazing makes soil less fertile is very hard, and the evidence that it can make soil more fertile if properly managed is anecdotal at best. Sixth, and most important, how is that relevant to us? Let us say there is a technology which can make use of vast areas of infertile land. Let us also say that it, as hard as it is to believe, uses animals and produces meat. So what? Meat you and I can buy is most likely produced in factory farms which destroy the environment with antibiotics, or, less likely, from poorly managed pastures. If you buy meat today, you are damaging the environment, and this is not controversial.). The other one is that it's supposedly easy to show that animals are incapable of higher cognitive functions. But, what requires little intelligence in humans often requires high intelligence in computers and vice versa, and who knows about animals (see The Moravec's Paradox).
Of course, the most common argument for eating meat is that it's supposedly natural. Now, how exactly people think first humans were hunting before the tools necessary for that were invented, I have no idea. Being able to eat meat probably helped our ancestors survive in some cases. Today, it does more harm than good, and that's clear to everyone who looks at the evidence.
Vitamin B12 deficiency wasn't a problem for our ancestors, because they didn't use to clean the fruits and vegetables before eating them, and many bacteria in dust also produce Vitamin B12. Today, it's probably not a good idea. The best source of the Vitamin B12 today is probably shellfish. Many foods, including some exotic plants, contain it, but our body doesn't absorb it from them well. Since around a third of the worlds' population is deficient in the Vitamin B12, it's probably a good idea to take it in form of vitamin pills. Some plants, like flax, contain a lot of Omega-3. However, it contains a different form of Omega-3 than our body needs, and our bodies aren't good at converting between different forms of Omega-3. So, it's not entirely clear if it's better to get Omega-3 from plants or from fish. The foods richest in proteins are beans and cheese. Of course, not all protein is equal, and what our body actually needs is to get all the essential amino acids from the protein in our diet. The soybeans contain enough of all the essential amino acids except the methionine, and the seeds rich in methionine are sesame seeds. However, it's also important not to eat too much methionine. In fact, the dangers of getting too much methionine probably outweigh the dangers of getting too little methionine. And milk of cows, and so do cheese and red meat, probably contains much more methionine than human body needs. Some nutritionists think that, paradoxically, the reason why vegetarians tend to have lower rates of heart disease is that many of them are "deficient" in methionine.
Contrary to popular belief, spinach isn't rich in iron at all. The best vegetarian source of iron is probably lentil (UPDATE on 12/12/2020: It is sometimes argued that red meat is a very good source of iron because of the heme-iron in it, present in no other food, and being absorbed a lot better than free iron. However, it is nearly universally agreed that heme-iron mixed with omega-6-acids, present in meat and even more so in processed meat, causes colon cancer. And, considering that anemia caused by the actual lack of iron is rather rare, a risk-benefit analysis is unlikely to favor ingesting heme-iron. ).
I hope this was enough information for you to understand vegetarianism.I am not arguing for forcing people to be vegetarian (that would probably just lead to black markets, and, also, what one speaks has way more effect on the world than what one eats), I just hope I've made some people think. Many people want to make the world a better place. But the right way to do that is to start from yourself. Do you strive not to cause unnecessary suffering of the innocent?

UPDATE on 28/07/2018: I've just written a parody showing how anti-vegetarian propaganda sounds like to vegetarians (and to everybody else, I suppose).

UPDATE on 04/08/2019: I've started a thread about vegetarian philosophy on a Latin forum, to see what people educated in philosophy and social sciences think about vegetarian philosophy. In case that website goes down, here is the short essay I wrote in the opening post, together with all the addenda and corrections (feel free to copy from me): Quid homines in hoc foro cogitant de vegetarianismo? Ego vegetarianista sum, ego credo carnem edere non licitum esse. Quomodo id licitum esse possit? Si carnem edimus, aliquis occidi debet. Id saepe non videmus, sed non licet nobis id ignorare. Et in Sina, multi homines canes edunt, easdem canes, quas nos amamus. In Helvetia etiam multi homines feles edunt. An canes et feles quodam modo differentes sunt a bubus et suibus, quas nos occidimus, ut carnem possimus edere? Forsan feles et canes sapientiores bubus et suibus sunt, sed boves et sues certe sapientiores liberis infantibus sunt, et quis neget infantes occidere non licitum esse? Sane, aliquae animalia non possunt vivere si ea animalia carnem non edant. Ergo quid? Mantides religiosae (magna insecta) etiam non possunt subolem habere, si eae non occidant et edant mares suos. An eius causa, quodam modo, licitum est mulieribus, quae homines sunt, suos maritos occidere?
Argumenta pro carne edenda, quae saepe dantur, sunt:
1) Carnis minime carus modus vitamini B12 acquirendi est.
Ita, multa animalia, boves inter alia, habent, in suis lactibus (intestinis), bacteria quae vitaminum B12 faciunt. Sed in carne, quae non cara est, vitaminum B12 paene non datur. Bubus, et aliis animalibus quae edimus, hodie semper antibiotica dantur. Ea antibiotica protegunt ea animalia a morbis, quos bacteria faciunt, quia ea animalia paene semper in parvis et sordidis spatiis sita sunt. Sed antibiotica etiam ea bacteria, quae vitaminum B12 faciunt, occidunt. Antibiotica enim et bona et mala bacteria occidunt. Carnis piscium vitaminum B12 continet, sed caro piscium multo carior (plures pecunias debes dare, ut carnem piscium possis emere) pilulis quae vitaminum B12 continent est.
Dare magnam summam antibioticorum animalibus etiam possibile facit, ut aliquae bacteria, quae et animalia et homines morbidos faciunt, ab antibioticis invulnerabilia fiant. Materia genetica, de invulnerabilitate ab antibioticis, in paene omnibus bacteriis data est, et ea potest activa fieri. Probabiliter, id fit iam hodie. Forsan multos morbos, quae hodie possumus curare antibioticis, in futura non poterimus curare... carnem edendi causa.
2) Edere magnam summam omega-3-acidi protegit a morbis cordis.
Simpliciter, hoc non est veritas. Evidentia huius non datur. Et si hoc veritas esset, non debeamus magnam summam carnis edere, debeamus magnam summam lini edere. Linum enim plenius omega-3-acidis quam caro est.
3) In optima diaeta, triens energiae ex proteinis acquisita est.
De veritate huius informationis loquitur id, quod lac hominum continet paene nulla proteina.
Spero, ut vos de his rebus cogitare faciam. Quod nos edimus, et quem aut quam nos edimus, afficit homines, et alia sentientia animalia, circa nos. Cum carnem emimus, facimus plures carnes fieri, id est, plura animalia ali et occidi. Leges de animalibus protegendis id non afficiunt.

UPDATE on 01/10/2019: I've just posted a video about vegetarianism on YouTube, you can see it here. If you can't, don't worry, all the information given there is also available on this website.

UPDATE on 17/10/2019: I've posted a response video in which I try to debunk the anti-vegetarian claims made by Nina Teicholz, you can see it here. In case it doesn't work, try this.

UPDATE on 01/04/2020: I've posted a response video in which I try to debunk the anti-vegetarian claims made by Peer Ederer, you can see it here. If you can't open it, try this.

UPDATE on 17/05/2020: I've posted a response to a blogger called DietDoctor who advocates meat-based diets, you can see it here.

UPDATE on 29/06/2020: I've made a quick meme about how absurd it is to blame people who eat bats for not caring about their diet causing dangerous pandemics, when most of the these days people do the same:
If eating bats is wrong because it causes coronavirus, eating factory-farmed meat is even more wrong because it causes superbacteria.
UPDATE on 27/12/2020: I've made a quick meme against the popular argument against vegetarianism "Most of the land can only feed us using animals.".
We should use cows to make use of the wild grasslands as land? So, you want cows to be poisoned by poisonous plants every now and then?