"We are in the midst of a speciesist era," Singer lectured an audience recently . "The ideology is that humans are more important than some other kind of being."

"I discovered the animal issue in 1970 and I wrote the first piece in 1973, I felt that this was something that other people need to consider and know about -equal consideration for animals," Singer, a professor in Australia and at Princeton University, told The Dodo in a phone interview. "What motivated me to write it was to get other people to think and talk about the issue." The issue was one that society at large had never really questioned before: whether or not animals had rights, feelings or deserved equal consideration.

He went on to describe the next set of moral and ethical issues facing not only philosophers, but humanity en masse: speciesism, a prejudice widely interpreted as a dominant group giving themselves a place in the universe where they matter more and justify certain actions that degrade other, less dominant species.

In 1973, the "New York Review of Books" published a cover essay by Australian philosopher Peter Singer titled "Animal Liberation." It was the first time that phrase appeared in print and the essay began with a simple political argument:

But that attitude is changing in large part due to the publication and proliferation of "Animal Liberation."

"Nobody talked about equality for animals 40 years ago, nobody even thought about it. Radical philosophers just assumed that human rights were for humans," Singer said. His paper, which he later expanded into a book by the same name, has been credited as one of the inspirations for the animal rights movement and a philosophy that is equal parts applied ethics and simplicity - that the dominant species (humans) should "avoid inflicting pain when you can, as a rule." Singer tackles complicated and controversial issues like euthanasia, poverty and war, but he's always been a fervent champion of animals by rallying against factory farming and animal testing and championing equality.

He's even been described by the New York Times as the most influential philosopher of our time. And 40 years after his paper was published, it's had a clear effect: "non-human personhood" is a phrase used freely in mainstream media outlets, the FBI has committed to tracking animal cruelty cases and McDonald's in Australia has agreed to only serve eggs from free-range chickens, all issues that may never have grazed the consciousness of the mainstream without Singer's influence. Singer talked with The Dodo about the progress that's been made, the missteps of captivity and what the future holds for the animal rights movement.

What do you think of Steve Wise's non-human personhood case?

I think great apes deserve rights, I hope they get them. I was a co-founder of the great ape project, in 1993 and it's taken time and energy, especially with the initiative of Steve Wise to build up the case. The fact that NYT gave Steve such prominence for his case shows there's a lot of interest in this area and maybe it is a realistic legal step but it's very hard to tell. These are decisions that will be made by judges in the American legal system. I can't predict, I hope it will happen, it will be philosophically justified.

Captivity is a major issue for animal rights. Should zoos exist for the sake preservation? Should zoos exist at all?

I think if a species is likely to become extinct in the wild and you can capture the animals humanely and recreate the physical and behavioral conditions, then could release them or their progeny in the wild, then that function of zoos is defensible.

What about SeaWorld, an institution that claims to be educational?

Basically SeaWorld teaches that we can capture animals and keep them in concrete pools, it doesn't teach people anything beyond that. There's incredible footage of cetaceans and we can learn a lot more in their natural environment and even if someone does learn something, is it worth the stress of the animal in exchange?

You've written extensively on speciesism and now there are conservation efforts saying some species simply can't be saved. Who and how can we make responsible decisions about allocating resources and time and responsibility, in terms of conservation?

If you have to make those decisions they will depend on the circumstances of each individual case. It depends on the scientist and the circumstance.

Some people have predicted that elephants will be extinct by 2025. Knowing that, is there a point to saving elephants? Or are they a relic of giant species past?

Not at this point with elephants, although the poaching is absolutely horrible. It's possible to maintain elephants in the wild and one of the major things would be to reduce the market for ivory and the same for rhinos. In terms of the economic impact on the local population, poaching brings in short term money but tourism will bring in long term sustainable income, so that poaching income can be replaced.

How did people - academic peers and the public - respond to "Animal Liberation" initially?

People loved their pets, that wasn't new. But the farm animal area, which is the biggest area of abuse, was a new topic of discussion. And nobody even understood what vegan was at the time. People just thought you might be from Vegas.

Have there been significant improvements in factory farming?

Definitely, most notably, in the European Union where they've begun by banning some of the worst forms of abuse on farms. Before you can change something like that you need to create awareness of what is being done to animals and what's wrong with what's being done to animals.

But how is it possible to consider factory farming issues improved when ag-gag legislation is passing?

It's not that factory farming isn't still there and an issue, it was pretty bad then and in some ways it's grown bigger and more dominant. But there are more alternatives and it's easier to get those alternatives.

What are the alternative options?

We haven't gotten rid of the problem and we need to go a lot further in understanding the problem. For animal and environmental reasons, change is happening but we're not over the hump yet. I think it's quite easy to easy to envision the abolition of factory farming. We don't need these animal products, they're not an efficient way of producing food. We're developing more and more alternatives all the time. Vegetarians and vegans are rising steadily. I guess a lot of people would just go to the grocery store and buy what they buy because they don't think ethically at all, it's just easier.

If you were to write "Animal Liberation" today, would you change the focus or add to the message? How?

I can't imagine writing it today because people's attitudes are very different and there's a lot of literature out there and I would wonder if it was needed? If I were starting out my career with what would most help animals, I'd probably go into the development of meat at a cellular level or plant-based products. Practical initiatives that will replace meat at an economically-competitive price point and that are cruelty-free will be the most powerful way to bring about change.