Ai Miei Genitori

ii Acknowledgements First, I would like to thank professors Paola Mello and Evelina Lamma for their gentle guidance, keen advice and patience. Their support and incitement have been fundamental for overcoming the most diicult times. I am grateful to professor Maurelio Boari for his uninterrupted encouragement, for creating a fertile environment for doing research and for having ooered me the opportunity to work in it. I would like to thank professor Antonis Kakas for the stimulating discussions we had on the topics of the integration of abduction and induction and for the enthusiasm he communicated me on the subject. Part of the work was done while I was visiting the University of Cyprus: my stay there was fruitful and enjoyable and I am grateful to Antonis Kakas for his hospitality. I would like to thank professor Luis Moniz Pereira for the interest he showed in my work and for many interesting discussions on the topics of knowledge representation in learning that were very inspiring for me. I would also like to thank all my friends at LIA, for having made more enjoyable my time at the department: Lastly, I would like to thank my parents Odette and Lamberto, my sister Daniela, my grandparents Odilla and Giuseppe for their love, support and faith in me.