Before starting to add numbers remember that we have seen that when counting you can only add things together if they have something in common.
1 egg add 1 eggs results in 2 eggs
1 apple add 1 apple results in 2 apples
1 bliken add 1 bliken results in 2 blikens.
Even if we do not know what a bliken is adding one of them to another one of them results in two of them. (Don’t bother to look in a dictionary for a bliken I made it up.)
However adding 1 egg and 1 apple can only be done in a list, as in a shopping list.
In the same way 1 bliken and 1 grifiat also cannot be added directly as blikens and grifiats are different.
The must be listed
So how about 1 Four add 1 Three?
Four and Three are different objects so can we add them?
Yes. They have something in common. What do they have in common? Ones.
Since one Four is the same as four Ones and one Three is the same as three Ones and four Ones add three Ones is, after counting, seven Ones which is one Seven and so one Four add one Three results in one Seven
or more lazily (and mathematicians can be lazy)
Four add Three results in Seven or even more lazily
4 + 3 = 7
Do mathematicians go through all that detailed precision to arrive at 4 + 3 = 7? The answer yes but even more so, they start with Nothing rather than One.