Further to Lal’s explanation;
There are 2 ways to attain nibbana.
1. By learning the dhamma by oneself. This is done by a Samma Sambuddha or a Pacceka Buddha
- By learning the Dhamma from a Buddha or a disciple of his. This done by an arahanth or sometimes referred to as a Shrawaka Buddha.
So these two methods are separate and will not overlap. The Buddha said that the Dhamma he learnt was something he had not heard or seen before. So even though he was ordained in previous Buddha Sasana’s he had not really understood this Dhamma.
Firewns said: but I am torn between a desire to escape samsara as soon as possible by becoming an arahant, and another desire to become a fully-enlightened teaching Buddha who teaches the Dhamma to alleviate the sufferings of sentient beings in an age when the laws of Dhamma are unknown.
Think of all the people who have not been exposed to the true Dhamma or have not understood it properly even when the Dhamma is there in the world like in the present time. So, becoming a Buddha is not the only way to teach Dhamma and help people from suffering. For example take the work Lal has been doing through this site, and many other theros and lay people are doing to spread the Dhamma. How many people would have benefited by work like this and reduced suffering.
Aspiring to become a Buddha is not some fleeting thought one may have during a period of time in samsara. It is not like the aspirations we may have had some time in our lives to become an astronaut or an Olympian or the President. The aspiration to become a Buddha is something much more deeper and deeply ingrained in such a person. Also a person aspiring to become a Buddha may not even know that he is aspiring for “Buddhahood” as he might be just wanting to learn the true nature of the world and teach it to others.
The term Bodhisatta is used to refer to the past lives of a Buddha after he has got “niyatha vivarana” to become a Buddha. It is a term or title given in retrospect. So a Bodhisatta, during that past life may not even know that he is destined to become a Buddha. Take the times that the Bodhisatta was born an animal even after getting niyatha vivarana.
I guess someone who is aspiring to become a Buddha can become an arahanth if he has not got “niyatha vivarana”.