What are the hog's defense, i.e., eye sight, smell?

Definitely their sense of smell is much higher than most people know.

They hear well too. And are intelligent.

I got a good lesson in hearing and intelligence a couple mornings ago. Knew the wind was wrong, but stalked a big boar anyway. I had to come out of brush 65 yards from him, and knew I had to be quick, before the wind at my back alerted him...

I wasn't quick enough to beat a 15mph wind though. As soon as I set up on my sticks, I didn't even have time to settle the crosshairs before he raised his head and bolted.

Their ability to see is better than that of humans. While they have a narrow cone of forward binocular vision (probably comparable to humans, they have something like 270-330 degree (I have seen both written) overall vision to detect movement and such, like many other animals such as deer. They see much better than we do at night. They can run through the woods in the dark without hitting trees.

1. Smell
2. Smell
3. Smell
4. Hearing
5. Vision

You can defeat their hearing by only moving when they have their snout buried in corn or rooting. They can't hear much over the noise that they themselves are making. You can defeat their vision by standing perfectly still when they are looking at you. If the wind is wrong, you can not defeat their sense of smell. I can sit in a box blind 100 yards away and the deer won't spook even if the wind is wrong. It does not work that way for pigs.

How to hunt pigs ^

I don't know, if their noses are down in the dirt, they don't seem to smell anything but dirt just like they aren't hearing anything over their own rooting.

I know folks say you can't beat a hog's nose, but I have managed to make a lot of approaches on hogs that were down wind of me. While they may have great ability, they don't always seem to use it.

Like humans, some ain’t as smart as others. Young and dumb seems to fit. We all seen a sounder come pouring out. How many times has it been big mamma leading?

But if the claim is that you can't defeat this sense or that sense, then really the issue of smartness isn't a salient issue.

How many times has it been big mama leading? You mean out in front? Not sure anyone keeps count. I have seem mama sows lead in their littles. I have seen the littles lead in the rest. I have seen them come in as a horde, almost like a single organism.

If you have a group of 10, statistically, there is a 90% chance that any one hog won't be leading when you see them.

We know from human studies that people are not on point 24/7, or in "Condition Yellow" as the tactical guys like to proclaim. Humans and animals alike suffer from inattentional blindness, even the smartest of them. Getting a hog in the crosshairs more easily than expected does not necessarily mean that the hog is less intelligent, only that it hasn't realized the threat.

I feel like pigs are a product of their environment, hunting pressure probably affects their behavior. Some easily sense my presence, even if completely covered and downwind , and won't enter the kill zone (feeding area) while I'm there. The walk the edge, look around snorting like they're mad I'm there and leave.
Some are dumb as heck. I've slow jogged behind sounders to catch up and got very close.
Once got within 10 feet of a boar in cover grunting at me.
Others can't get within 150 yards and they're running.
I try to stalk to within 30 yards with bow, but usually I spook them at 40 or 50 yds, still learning.
I like a strong wind, makes noise and seems to disperse scent better.

Once had a sounder run in front of me. They were moving fast so thinking I could catch a straggler I ran up and waited, and a sole sow came 50 yds behind the sounder. I creeped closer with help of a few trees and she stopped in the tall grass - I believe she either saw or smelled me and thought she thought she was hidden by the grass so stood very still, kind of stupid but this allowed a bow shot.
Moral of the story is pigs are different and just have to get out there to see what yours are like but more than likely they're going to be smart ones.