Probably, likely, possibly と perhaps, maybe との違い

抄録

Many English-Japanese dictionaries and English-usage dictionaries in Japan classify ‘probably’, ‘likely’, ‘possibly’, ‘perhaps’ and ‘maybe’ into the same group, all of which indicate the possibility of occurrence of something. Some of them further show their respective percentages.
In this paper I will show that these words are divided into two groups according to whether they express the speaker's certainty or not. ‘Probably’, ‘likely’ and ‘possibly’ indicate that the speaker is certain that something will happen, while ‘perhaps’ and ‘maybe’ indicate that he is not certain. Some syntactic features such as the following will support this analysis: the former group words can be modified by ‘very’ and ‘quite’, and take comparative and superlative forms, and the latter group words can modify a number with the meaning ‘about’.