The intervals between the numbers in the sequence are the numbers that are not in the sequence.
Start with the number 1.
Because the number 2 is not in the sequence (all that we have so far is the number 1), we use that as the next interval. Adding 2 to 1 gives 3. So 3 is in.
The next number that is not present in the series is 4. Adding that to 3 gives us 7.
Because 5 has been skipped over, and is not in the sequence, we add that to 7, to get 12.
6 isn't there either, so we add that to 12 to get 18.
The next missing number is 8. Adding that to 18 gives us 26...
... and so on. The sequence and its inverse set define each other in a strange, recursive way. Gave me goose bumps when I first realized what was going on.