What about crows?
I cannot agree with this. Maturity has a very clear meaning: it is when a human (or animal) has developed enough to be able to sexually reproduce. That people use the word wrongly in everyday conversation is something else.
As for what you write about mammals, I completely agree. Just looking at them and interacting with them in real life and it's clear that they're our brothers.
Physical maturity is irrelevant when it comes to mental decisions of how to apply that physical maturity.
That's what cognitive dissonance is. Giving one's senses (in this case the taste buds or the ego 'I' or the mind) more importance than something factual or apparent.
> I am not convinced about the rest of the animal kingdom.
If you take consciousness to mean intelligence, then sure. If you take consciousness to be separate from intelligence and senses, being the energy or force that brings forth the signs of what we call 'life', then I see no reason to believe that every single living entity has consciousness.
The only difference is in the type of body provided to that conscious living entity. Just like how some humans are born without sight, some without hearing, some without the ability to speak. It doesn't mean they're less conscious, they still are an observer observing things through their senses.
Unless and until one can come up with a clear definition of consciousness (you don't need to, just read nyāya philosophy and you'll save thousands of years of efforts), they will not be able to get over their biases based on unintelligent thoughts and cognitive dissonance.
I think humans had exactly the same cognitive dissonance about slavery and racial-based discrimination (in all it's forms) for most of our history.
It wasn't so much that their HATE was so powerful that it superseded logical consistency, but rather the CONVENIENCE of this discrimination was so attractive. Classism is the same way.
We make up ipso facto justifications to continue to perpetuate inequality and exploitation that benefits us. The most recent version of this is globalization and all the arguments the west makes for how our reliance on 3rd world labour improves their livelihood. (It does; it's also more complex than that)
I'm not going to pretend that I WANT to be vegetarian or WANT to align my logical ethics with my actions. I clearly do not, I find eating meat too convenient and brings me too much joy.
But I do want to want to...
> I find eating meat too convenient and brings me too much joy.
Yes but then you have the question: Is your convenience worth more than someone's life? Does your joy justify someone's throat being cut?
I'd say that this is not the same as slavery or racial-based discrimination since this is someone's aliveness at stake and the only crime that they committed was being in a different body than you. This is not about forcing them to work or saying mean things, but immense pain and extreme suffering which causes the consciousness to leave the body.
> But I do want to want to...
That shows that your intelligence agrees but the mind, being connected to the senses, wants to divert attention towards sense pleasures.
Sāṁkhya philosophy would say that a human whose intelligence is stolen by the senses is no better than an animal: eating, sleeping, mating and defending. Now, I'm not saying this to be judgemental but this metaphysical system gives you a lens to see and act in the world.
What then separates us from being a human if we're no better than a pig rolling in dirt if all we know is running after one temporary sense pleasure to the next? and what if somehow after you die, your consciousness takes the body of a similar animal? (Unless of course you believe in Creatio ex nihilo).
These are questions to ask in life. The human birth is special because while being similar to literally every other species, this birth is the only kind of birth that allows one to philosophize and ask important questions in life.
Being a vegetarian is easy in today's world, especially if you're privileged to be earning more money than 5 families in South Asia combined. All it really takes, is accepting intelligence's advice, instead of letting the mind (which controls the senses) reign.
For example, everything we associate with the magical age of 18 years old. "Adulthood", sexual consent, voting, drinking, smoking, conscription all begin on an arbitrary day and we take no consideration of the reality of the maturity of the person.
(Not to mention that "maturity" is entirely cultural and contextual. A harsher world (like one that most of humanity existed in for our entire history) causes earlier maturity. A gentler one (that we are obviously all trying to create) delays it.
So with animals, I see the spectrum, and I see that we are seeking to set a line...somewhere. Some set it just after humans. Humans are special, and everyone else is fair game. Vegans also have a pretty clear line.
Everything else, vegetarians included, are pretty "fuzzy". I'm an omnivore myself, but if you asked me for a concrete logical reason why my personal ethics allow me to eat pig but not dog, I don't think I could give you a ethically consistent one other than "pigs are delicious, and dogs are my friends".
Personally, I do believe that the same way that our generation asked our grandparents how they could've been ok with the racial discrimination & segregation that was ubiquitous in their youth, I think my grandchildren will ask me the same about my consumption of meat.
The reasons will likely be a mixture of climate impact (species of fish going extinct), and cost (because of the energy cost required to create pigs and cows). Society will form an ethical consensus about why it's not appropriate to eat animals to help reduce WW energy costs the same way that early people in the Levant found it helpful to create rules about not consuming pigs or shellfish (for sanitary reasons).
And there'll be "scandals" about rich people that have secret animal farms similar to what we have today about finding out about billionaires in dubai basically having secret house slaves. (AKA, outrage, but no meaningful change)
BTW from where I stand, it seems fairly reasonable to deduce that all mammals are in some way conscious/sentient, and have intelligence comparable to our own. Mammals play, mourn their dead, have a common signal that generally causes us to protect the young of all mammal species and find them cute.
I also think that there is a strong case for a very unique type of intelligence and conscience in Cephalopods. I have personally taken a stance to not eat cephalopods. Because I think they are in greater danger of going extinct vs. mammals (since their stock counts are so invisible to humanity), and I have decided (subjectively) that consuming a distinct type of intelligence from my own constitutes a form of crime. I don't know if I can defend that in court though.
I am not convinced about the rest of the animal kingdom.