As generally interpreted this line leaves much to be desired because of its unwieldy construction. It is true that est ut plus a form of possum is a common periphrasis in Lucretius (for examples v. Munro, 4th edition, note on 1. 620). Quite common also is Lucretius' use of the infinitive as a nominative substantive (examples cited by Munro, note on 1. 331). But the combination of fieri mirarier with the already periphrastic erat ut posset results in ‘a very clumsy phrase.… The line should be regarded as one which Lucretius might have improved on revision’ (Bailey).